README.md

# The Concise TypeScript Book

The Concise TypeScript Book provides a comprehensive and succinct overview of TypeScript's capabilities. It offers clear explanations covering all aspects found in the latest version of the language, from its powerful type system to advanced features. Whether you're a beginner or an experienced developer, this book is an invaluable resource to enhance your understanding and proficiency in TypeScript.

This book is completely Free and Open Source.

## Translations

This book has been translated into several language versions, including:

* [Chinese](./README-zh_CN.md)

## Downloads

You can also download the Epub version here:

<https://github.com/gibbok/typescript-book/tree/main/downloads>

## Table of content

<!-- markdownlint-disable MD004 -->
- [The Concise TypeScript Book](#the-concise-typescript-book)
  - [Translations](#translations)
  - [Downloads](#downloads)
  - [Table of content](#table-of-content)
  - [Introduction](#introduction)
  - [About the author](#about-the-author)
  - [TypeScript an introduction](#typescript-an-introduction)
    - [What is TypeScript?](#what-is-typescript)
    - [Why TypeScript?](#why-typescript)
    - [TypeScript and JavaScript](#typescript-and-javascript)
    - [TypeScript code generation](#typescript-code-generation)
    - [Modern JavaScript Now (Downleveling)](#modern-javascript-now-downleveling)
  - [Getting Started With TypeScript](#getting-started-with-typescript)
    - [Installation](#installation)
    - [Configuration](#configuration)
    - [TypeScript configuration file ​​tsconfig.json](#typescript-configuration-file-tsconfigjson)
      - [target](#target)
      - [lib](#lib)
      - [strict](#strict)
      - [module](#module)
      - [moduleResolution](#moduleresolution)
      - [esModuleInterop](#esmoduleinterop)
      - [jsx](#jsx)
      - [skipLibCheck](#skiplibcheck)
      - [files](#files)
      - [include](#include)
      - [exclude](#exclude)
    - [Migration to TypeScript Advice](#migration-to-typescript-advice)
  - [Exploring the Type System](#exploring-the-type-system)
    - [The TypeScript language service](#the-typescript-language-service)
    - [Structural Typing](#structural-typing)
    - [TypeScript Fundamental Comparison Rules](#typescript-fundamental-comparison-rules)
    - [Types as sets](#types-as-sets)
    - [Assign a type: Type Declarations and Type Assertions](#assign-a-type-type-declarations-and-type-assertions)
      - [Type Declaration](#type-declaration)
      - [Type Assertion](#type-assertion)
      - [Non-null assertion](#non-null-assertion)
      - [Ambient Declarations](#ambient-declarations)
    - [Property Checking and Excess Property Checking](#property-checking-and-excess-property-checking)
    - [Weak Types](#weak-types)
    - [Strict Object Literal Checking (Freshness)](#strict-object-literal-checking-freshness)
    - [Type Inference](#type-inference)
    - [More advanced inferences](#more-advanced-inferences)
    - [Type Widening](#type-widening)
    - [Const](#const)
      - [const modifier on type parameters](#const-modifier-on-type-parameters)
    - [Explicit Type Annotation](#explicit-type-annotation)
    - [Const assertion](#const-assertion)
    - [Type Narrowing](#type-narrowing)
      - [Conditions](#conditions)
      - [Throwing or returning](#throwing-or-returning)
      - [Discriminated union](#discriminated-union)
      - [User-defined type guards](#user-defined-type-guards)
  - [Primitive Types](#primitive-types)
    - [string](#string)
    - [boolean](#boolean)
    - [number](#number)
    - [bigInt](#bigint)
    - [symbol](#symbol)
    - [null and undefined](#null-and-undefined)
    - [Array](#array)
    - [any](#any)
  - [Type Annotations](#type-annotations)
  - [Optional Properties](#optional-properties)
  - [Readonly Properties](#readonly-properties)
  - [Index Signatures](#index-signatures)
  - [Extending Types](#extending-types)
  - [Literal Types](#literal-types)
  - [Literal Inference](#literal-inference)
  - [strictNullChecks](#strictnullchecks)
  - [Non-null Assertion Operator (Postfix !)](#non-null-assertion-operator-postfix-)
  - [Enums](#enums)
    - [Numeric enums](#numeric-enums)
    - [String enums](#string-enums)
    - [Constant enums](#constant-enums)
    - [Reverse mapping](#reverse-mapping)
    - [Ambient enums](#ambient-enums)
    - [Computed and constant members](#computed-and-constant-members)
  - [Narrowing](#narrowing)
    - [typeof type guards](#typeof-type-guards)
    - [Truthiness narrowing](#truthiness-narrowing)
    - [Equality narrowing](#equality-narrowing)
    - [In operator narrowing](#in-operator-narrowing)
    - [instanceof narrowing](#instanceof-narrowing)
  - [Assignments](#assignments)
  - [Control flow analysis](#control-flow-analysis)
  - [type predicates](#type-predicates)
  - [Discriminated unions](#discriminated-unions)
  - [The never type](#the-never-type)
  - [Exhaustiveness checking](#exhaustiveness-checking)
  - [Object Types](#object-types)
  - [Tuple Type](#tuple-type)
  - [Fixed length tuple](#fixed-length-tuple)
  - [Union Type](#union-type)
  - [Intersection Types](#intersection-types)
  - [Type Indexing](#type-indexing)
  - [Type from Value](#type-from-value)
  - [Type from Func Return](#type-from-func-return)
  - [Type from Module](#type-from-module)
  - [Mapped types](#mapped-types)
  - [Conditional Types](#conditional-types)
  - [Distributive conditional types](#distributive-conditional-types)
  - [“infer” Type inference in conditional types](#infer-type-inference-in-conditional-types)
  - [Predefined conditional types](#predefined-conditional-types)
  - [Template Union Types](#template-union-types)
  - [Any type](#any-type)
  - [Unknown type](#unknown-type)
  - [Void type](#void-type)
  - [Never type](#never-type)
  - [Interface and Type](#interface-and-type)
    - [Common Syntax](#common-syntax)
    - [Basic types](#basic-types)
    - [Objects and interfaces](#objects-and-interfaces)
    - [Union and intersection types](#union-and-intersection-types)
  - [Built-in Type Primitives](#built-in-type-primitives)
  - [Common Built-in JS Objects](#common-built-in-js-objects)
  - [Overloads](#overloads)
  - [Get \& Set](#get--set)
  - [Merging and Extension](#merging-and-extension)
  - [Differences between Type and Interface](#differences-between-type-and-interface)
  - [Class](#class)
    - [Class Common Syntax](#class-common-syntax)
    - [Constructor](#constructor)
    - [Private and Protected Constructors](#private-and-protected-constructors)
    - [Access modifiers](#access-modifiers)
    - [Auto-Accessors in Classes](#auto-accessors-in-classes)
    - [this](#this)
    - [Parameter Properties](#parameter-properties)
    - [Abstract Classes](#abstract-classes)
    - [With Generics](#with-generics)
    - [Decorators](#decorators)
      - [Class decorators](#class-decorators)
      - [Property Decorator](#property-decorator)
      - [Method Decorator](#method-decorator)
      - [Getter and Setter Decorators](#getter-and-setter-decorators)
    - [Inheritance](#inheritance)
    - [Statics](#statics)
    - [Property initialization](#property-initialization)
    - [Method overloading](#method-overloading)
  - [Generics](#generics)
    - [Generic Type](#generic-type)
    - [Generic Classes](#generic-classes)
    - [Generic Constraints](#generic-constraints)
    - [Generic contextual narrowing](#generic-contextual-narrowing)
  - [Erased Structural Types](#erased-structural-types)
  - [Namespacing](#namespacing)
  - [Symbols](#symbols)
  - [Triple-Slash Directives](#triple-slash-directives)
  - [Type Manipulation](#type-manipulation)
    - [Creating Types from Types](#creating-types-from-types)
    - [Indexed Access Types](#indexed-access-types)
    - [Utility Types](#utility-types)
      - [Awaited\<T\>](#awaitedt)
      - [Partial\<T\>](#partialt)
      - [Required\<T\>](#requiredt)
      - [Readonly\<T\>](#readonlyt)
      - [Record\<K, T\>](#recordk-t)
      - [Pick\<T, K\>](#pickt-k)
      - [Omit\<T, K\>](#omitt-k)
      - [Exclude\<T, U\>](#excludet-u)
      - [Extract\<T, U\>](#extractt-u)
      - [NonNullable\<T\>](#nonnullablet)
      - [Parameters\<T\>](#parameterst)
      - [ConstructorParameters\<T\>](#constructorparameterst)
      - [ReturnType\<T\>](#returntypet)
      - [InstanceType\<T\>](#instancetypet)
      - [ThisParameterType\<T\>](#thisparametertypet)
      - [OmitThisParameter\<T\>](#omitthisparametert)
      - [ThisType\<T\>](#thistypet)
      - [Uppercase\<T\>](#uppercaset)
      - [Lowercase\<T\>](#lowercaset)
      - [Capitalize\<T\>](#capitalizet)
      - [Uncapitalize\<T\>](#uncapitalizet)
  - [Others](#others)
    - [Errors and Exception Handling](#errors-and-exception-handling)
    - [Asynchronous Language Features](#asynchronous-language-features)
    - [Iterators and Generators](#iterators-and-generators)
    - [TsDocs JSDoc Reference](#tsdocs-jsdoc-reference)
    - [@types](#types)
    - [JSX](#jsx-1)
    - [ES6 Modules](#es6-modules)
    - [ES7 exponentiation operator](#es7-exponentiation-operator)
    - [The for-await-of Statement](#the-for-await-of-statement)
    - [New.target](#newtarget)
    - [Dynamic Import Expressions](#dynamic-import-expressions)
    - [“tsc –watch”](#tsc-watch)
    - [Definite Assignment Assertions (!)](#definite-assignment-assertions-)
    - [Defaulted declarations](#defaulted-declarations)
    - [“const“ assertions](#const-assertions)
    - [Optional Chaining](#optional-chaining)
    - [Nullish coalescing operator (??)](#nullish-coalescing-operator-)
    - [Template Literal Types](#template-literal-types)
    - [Function overloading](#function-overloading)
    - [Recursive Conditional Types](#recursive-conditional-types)
    - [ECMAScript Module Support in Node.js](#ecmascript-module-support-in-nodejs)
    - [Assertion Functions](#assertion-functions)
    - [Variadic Tuple Types](#variadic-tuple-types)
    - [Boxed types](#boxed-types)
    - [Key Remapping in Mapped Types](#key-remapping-in-mapped-types)
    - [Covariance and Contravariance in TypeScript](#covariance-and-contravariance-in-typescript)
      - [Optional Variance Annotations for Type Parameters](#optional-variance-annotations-for-type-parameters)
    - [Symbol and Template String Pattern Index Signatures](#symbol-and-template-string-pattern-index-signatures)
    - [The satisfies Operator](#the-satisfies-operator)
<!-- markdownlint-enable MD004 -->
## Introduction

Welcome to The Concise TypeScript Book! This guide equips you with essential knowledge and practical skills for effective TypeScript development. Discover key concepts and techniques to write clean, robust code. Whether you're a beginner or an experienced developer, this book serves as both a comprehensive guide and a handy reference for leveraging TypeScript's power in your projects.

## About the author

Simone Poggiali is an experienced Senior Front-end Developer with a passion for writing professional-grade code since the 90s. Throughout his international career, he has contributed to numerous projects for a wide range of clients, from startups to large organizations. Notable companies such as HelloFresh, Siemens, O2, and Leroy Merlin have benefited from his expertise and dedication.

You can reach Simone Poggiali on the following platforms:

* LinkedIn: <https://www.linkedin.com/in/simone-poggiali>
* GitHub: <https://github.com/gibbok>
* Twitter: <https://twitter.com/gibbok_coding>
* Email: gibbok.coding📧gmail.com

## TypeScript an introduction

### What is TypeScript?

TypeScript is a strongly typed programming language that builds on JavaScript. It was originally designed by Anders Hejlsberg in 2012 and is currently developed and maintained by Microsoft as an open source project.

TypeScript compiles to JavaScript and can be executed in any JavaScript engine (e.g., a browser or server Node.js).

TypeScript supports multiple programming paradigms such as functional, generic, imperative, and object-oriented. TypeScript is neither an interpreted nor a compiled language.

### Why TypeScript?

TypeScript is a strongly typed language that helps prevent common programming mistakes and avoid certain kinds of run-time errors before the program is executed.

A strongly typed language allows the developer to specify various program constraints and behaviors in the data type definitions, facilitating the ability to verify the correctness of the software and prevent defects. This is especially valuable in large-scale applications.

Some of the benefits of TypeScript:

* Static typing, optionally strongly typed
* Type Inference
* Access to ES6 and ES7 features
* Cross-Platform and Cross-browser Compatibility
* Tooling support with IntelliSense

### TypeScript and JavaScript

TypeScript is written in `.ts` or `.tsx` files, while JavaScript files are written in `.js` or `.jsx`.

Files with the extension `.tsx` or `.jsx` can contain JavaScript Syntax Extension JSX, which is used in React for UI development.

TypeScript is a typed superset of JavaScript (ECMAScript 2015) in terms of syntax. All JavaScript code is valid TypeScript code, but the reverse is not always true.

For instance, consider a function in a JavaScript file with the `.js` extension, such as the following:

<!-- skip -->
```typescript
const sum = (a, b) => a + b;
```

The function can be converted and used in TypeScript by changing the file extension to `.ts`. However, if the same function is annotated with TypeScript types, it cannot be executed in any JavaScript engine without compilation. The following TypeScript code will produce a syntax error if it is not compiled:

<!-- skip -->
```typescript
const sum = (a: number, b: number): number => a + b;
```

TypeScript was designed to detect possible exceptions that can occur at runtime during compilation time by having the developer define the intent with type annotations. In addition, TypeScript can also catch issues if no type annotation is provided. For instance, the following code snippet does not specify any TypeScript types:

<!-- skip -->
```typescript
const items = [{ x: 1 }, { x: 2 }];
const result = items.filter(item => item.y);
```

In this case, TypeScript detects an error and reports:

```text
Property 'y' does not exist on type '{ x: number; }'.
```

TypeScript's type system is largely influenced by the runtime behavior of JavaScript. For example, the addition operator (+), which in JavaScript can either perform string concatenation or numeric addition, is modeled in the same way in TypeScript:

```typescript
const result = '1' + 1; // Result is of type string
```

The team behind TypeScript has made a deliberate decision to flag unusual usage of JavaScript as errors. For instance, consider the following valid JavaScript code:

<!-- skip -->
```typescript
const result = 1 + true; // In JavaScript, the result is equal 2
```

However, TypeScript throws an error:

Operator '+' cannot be applied to types 'number' and 'boolean'.

This error occurs because TypeScript strictly enforces type compatibility, and in this case, it identifies an invalid operation between a number and a boolean.

### TypeScript code generation

The TypeScript compiler has two main responsibilities: checking for type errors and compiling to JavaScript. These two processes are independent of each other. Types do not affect the execution of the code in a JavaScript engine, as they are completely erased during compilation. TypeScript can still output JavaScript even in the presence of type errors.
Here is an example of TypeScript code with a type error:

<!-- skip -->
```typescript
const add = (a: number, b: number): number => a + b;
const result = add('x', 'y'); // Argument of type 'string' is not assignable to parameter of type 'number'.
```

However, it can still produce executable JavaScript output:

<!-- skip -->
```typescript
'use strict';
const add = (a, b) => a + b;
const result = add('x', 'y'); // xy
```

It is not possible to check TypeScript types at runtime. For example:

<!-- skip -->
```typescript
interface Animal {
    name: string;
}
interface Dog extends Animal {
    bark: () => void;
}
interface Cat extends Animal {
    meow: () => void;
}
const makeNoise = (animal: Animal) => {
    if (animal instanceof Dog) {
        // 'Dog' only refers to a type, but is being used as a value here.
        // ...
    }
};
```

As the types are erased after compilation, there is no way to run this code in JavaScript. To recognize types at runtime, we need to use another mechanism. TypeScript provides several options, with a common one being "tagged union". For example:

```typescript
interface Dog {
    kind: 'dog'; // Tagged union
    bark: () => void;
}
interface Cat {
    kind: 'cat'; // Tagged union
    meow: () => void;
}
type Animal = Dog | Cat;

const makeNoise = (animal: Animal) => {
    if (animal.kind === 'dog') {
        animal.bark();
    } else {
        animal.meow();
    }
};

const dog: Dog = {
    kind: 'dog',
    bark: () => console.log('bark'),
};
makeNoise(dog);
```

The property "kind" is a value that can be used at runtime to distinguish between objects in JavaScript.

It is also possible for a value at runtime to have a type different from the one declared in the type declaration. For instance, if the developer has misinterpreted an API type and annotated it incorrectly.

TypeScript is a superset of JavaScript, so the "class" keyword can be used as a type and value at runtime.

```typescript
class Animal {
    constructor(public name: string) {}
}
class Dog extends Animal {
    constructor(public name: string, public bark: () => void) {
        super(name);
    }
}
class Cat extends Animal {
    constructor(public name: string, public meow: () => void) {
        super(name);
    }
}
type Mammal = Dog | Cat;

const makeNoise = (mammal: Mammal) => {
    if (mammal instanceof Dog) {
        mammal.bark();
    } else {
        mammal.meow();
    }
};

const dog = new Dog('Fido', () => console.log('bark'));
makeNoise(dog);
```

In JavaScript, a "class" has a "prototype" property, and the "instanceof" operator can be used to test if the prototype property of a constructor appears anywhere in the prototype chain of an object.

TypeScript has no effect on runtime performance, as all types will be erased. However, TypeScript does introduce some build time overhead.

### Modern JavaScript Now (Downleveling)

TypeScript can compile code to any released version of JavaScript since ECMAScript 3 (1999). This means that TypeScript can transpile code from the latest JavaScript features to older versions, a process known as downleveling. This allows the usage of modern JavaScript while maintaining maximum compatibility with older runtime environments.

It's important to note that during transpilation to an older version of JavaScript, TypeScript may generate code that could incur a performance overhead compared to native implementations.

Here are some of the modern JavaScript features that can be used in TypeScript:

* ECMAScript modules instead of AMD-style "define" callbacks or CommonJS "require" statements.
* Classes instead of prototypes.
* Variables declaration using "let" or "const" instead of "var".
* "for-of" loop or ".forEach" instead of the traditional "for" loop.
* Arrow functions instead of function expressions.
* Destructuring assignment.
* Shorthand property/method names and computed property names.
* Default function parameters.

By leveraging these modern JavaScript features, developers can write more expressive and concise code in TypeScript.

## Getting Started With TypeScript

### Installation

Visual Studio Code provides excellent support for the TypeScript language but does not include the TypeScript compiler. To install the TypeScript compiler, you can use a package manager like npm or yarn:

```shell
npm install typescript --save-dev
```

or

```shell
yarn add typescript --dev
```

Make sure to commit the generated lockfile to ensure that every team member uses the same version of TypeScript.

To run the TypeScript compiler, you can use the following commands

```shell
npx tsc
```

or

```shell
yarn tsc
```

It is recommended to install TypeScript project-wise rather than globally, as it provides a more predictable build process. However, for one-off occasions, you can use the following command:

```shell
npx tsc
```

or installing it globally:

```shell
npm install -g typescript
```

If you are using Microsoft Visual Studio, you can obtain TypeScript as a package in NuGet for your MSBuild projects. In the NuGet Package Manager Console, run the following command:

```shell
Install-Package Microsoft.TypeScript.MSBuild
```

During the TypeScript installation, two executables are installed: "tsc" as the TypeScript compiler and "tsserver" as the TypeScript standalone server. The standalone server contains the compiler and language services that can be utilized by editors and IDEs to provide intelligent code completion.

Additionally, there are several TypeScript-compatible transpilers available, such as Babel (via a plugin) or swc. These transpilers can be used to convert TypeScript code into other target languages or versions.

### Configuration

TypeScript can be configured using the tsc CLI options or by utilizing a dedicated configuration file called tsconfig.json placed in the root of the project.

To generate a tsconfig.json file prepopulated with recommended settings, you can use the following command:

```shell
tsc --init
```

When executing the `tsc` command locally, TypeScript will compile the code using the configuration specified in the nearest tsconfig.json file.

Here are some examples of CLI commands that run with the default settings:

```shell
tsc main.ts // Compile a specific file (main.ts) to JavaScript
tsc src/*.ts // Compile any .ts files under the 'src' folder to JavaScript
tsc app.ts util.ts --outfile index.js // Compile two TypeScript files (app.ts and util.ts) into a single JavaScript file (index.js)
```

### TypeScript configuration file ​​tsconfig.json

A tsconfig.json file is used to configure the TypeScript Compiler (tsc). Usually, it is added to the root of the project, together with the `package.json` file.

Notes:

* tsconfig.json accepts comments even if it is in json format.
* It is advisable to use this configuration file instead of the command-line options.

At the following link you can find the complete documentation and its schema:
<https://www.typescriptlang.org/tsconfig>
<http://json.schemastore.org/tsconfig>

The following represents a list of the common and useful configurations:

#### target

The "target" property is used to specify which version of JavaScript ECMAScript version your TypeScript should emit/compile into. For modern browsers ES6 is a good option, for older browsers, ES5 is recommended.

#### lib

The "lib" property is used to specify which library files to include at compilation time. TypeScript automatically includes APIs for features specified in the "target" property, but it is possible to omit or pick specific libraries for particular needs. For instance, if you are working on a server project, you could exclude the "DOM" library, which is useful only in a browser environment.

#### strict

The "strict" property enables stronger guarantees and enhances type safety. It is advisable to always include this property in your project's tsconfig.json file. Enabling the "strict" property allows TypeScript to:

* Emit code using "use strict" for each source file.
* Consider "null" and "undefined" in the type checking process.
* Disable the usage of the "any" type when no type annotations are present.
* Raise an error on the usage of the "this" expression, which would otherwise imply the "any" type.

#### module

The "module" property sets the module system supported for the compiled program. During runtime, a module loader is used to locate and execute dependencies based on the specified module system.

The most common module loaders used in JavaScript are Node.js CommonJS for server-side applications and RequireJS for AMD modules in browser-based web applications. TypeScript can emit code for various module systems, including UMD, System, ESNext, ES2015/ES6, and ES2020.

Note: The module system should be chosen based on the target environment and the module loading mechanism available in that environment.

#### moduleResolution

The "moduleResolution" property specifies the module resolution strategy. Use “node” for modern TypeScript code, the “classic” strategy is used only for old versions of TypeScript (before 1.6).

#### esModuleInterop

The "esModuleInterop" property allows import default from CommonJS modules that did not export using the “default” property, this property provides a shim to ensure compatibility in the emitted JavaScript. After enabling this option we can use “import MyLibrary from ‘my-library’” instead of “import \* as MyLibrary from ‘my-library’”.

#### jsx

The "jsx" property applies only to .tsx files used in ReactJS and controls how JSX constructs are compiled into JavaScript. A common option is “preserve” which will compile to a .jsx file keeping unchanged the JSX so it can be passed to different tools like Babel for further transformations.

#### skipLibCheck

The "skipLibCheck'' property will prevent TypeScript from type-checking the entire imported third-party packages. This property will reduce the compile time of a project. TypeScript will still check your code against the type definitions provided by these packages.

#### files

The "files" property indicates to the compiler a list of files that must always be included in the program.

#### include

<!-- markdownlint-disable MD049 -->
The "include" property indicates to the compiler a list of files that we would like to include. This property allows glob-like patterns, such as "\*_" for any subdirectory, "_" for any file name, and "?" for optional characters.
<!-- markdownlint-enable MD049 -->

#### exclude

The "exclude" property indicates to the compiler a list of files that should not be included in the compilation. This can include files such as "node_modules" or test files.
Note: tsconfig.json allows comments.

### Migration to TypeScript Advice

For large projects, it is recommended to adopt a gradual transition where TypeScript and JavaScript code will initially coexist. Only small projects can be migrated to TypeScript in one go.

The first step of this transition is to introduce TypeScript into the build chain process. This can be done by using the "allowJs" compiler option, which permits .ts and .tsx files to coexist with existing JavaScript files. As TypeScript will fall back to a type of "any" for a variable when it cannot infer the type from JavaScript files, it is recommended to disable "noImplicitAny" in your compiler options at the beginning of the migration.

The second step is to ensure that your JavaScript tests work alongside TypeScript files so that you can run tests as you convert each module. If you are using Jest, consider using `ts-jest`, which allows you to test TypeScript projects with Jest.

The third step is to include type declarations for third-party libraries in your project. These declarations can be found either bundled or on DefinitelyTyped. You can search for them using <https://www.typescriptlang.org/dt/search> and install them using

```shell
npm install --save-dev @types/package-name or yarn add --dev @types/package-name.
```

The fourth step is to migrate module by module with a bottom-up approach, following your Dependency Graph starting with the leaves. The idea is to start converting Modules that do not depend on other Modules. To visualize the dependency graphs, you can use the `madge` tool.

Good candidate modules for these initial conversions are utility functions and code related to external APIs or specifications. It is possible to automatically generate TypeScript type definitions from Swagger contracts, GraphQL or JSON schemas to be included in your project.

When there are no specifications or official schemas available, you can generate types from raw data, such as JSON returned by a server. However, it is recommended to generate types from specifications instead of data to avoid missing edge cases.

During the migration, refrain from code refactoring and focus only on adding types to your modules.

The fifth step is to enable "noImplicitAny," which will enforce that all types are known and defined, providing a better TypeScript experience for your project.

During the migration, you can use the `@ts-check` directive, which enables TypeScript type checking in a JavaScript file. This directive provides a loose version of type checking and can be initially used to identify issues in JavaScript files. When `@ts-check` is included in a file, TypeScript will try to deduce definitions using JSDoc-style comments. However, consider using JSDoc annotations only at a very early stage of the migration.

Consider keeping the default value of `noEmitOnError` in your tsconfig.json as false. This will allow you to output JavaScript source code even if errors are reported.

## Exploring the Type System

### The TypeScript language service

The TypeScript language service, also known as tsserver, offers various features such as error reporting, diagnostics, compile-on-save, renaming, go to definition, completion lists, signature help, and more. It is primarily used by integrated development environments (IDEs) to provide IntelliSense support. It seamlessly integrates with Visual Studio Code and is utilized by tools like Conquer of Completion (Coc).

Developers can leverage a dedicated API and create their own custom language service plugins to enhance the TypeScript editing experience. This can be particularly useful for implementing special linting features or enabling auto-completion for a custom templating language.

<!-- markdownlint-disable MD044 -->
An example of a real-world custom plugin is "typescript-styled-plugin," which provides syntax error reporting and IntelliSense support for CSS properties in styled components.
<!-- markdownlint-enable MD044 -->

For more information and quick start guides, you can refer to the official TypeScript Wiki on GitHub: <https://github.com/microsoft/TypeScript/wiki/>

### Structural Typing

TypeScript is based on a structural type system. This means that the compatibility and equivalence of types are determined by the type's actual structure or definition, rather than its name or place of declaration, as in nominative type systems like C# or C.

TypeScript's structural type system was designed based on how JavaScript's dynamic duck typing system works during runtime.

The following example is valid TypeScript code. As you can observe, "X" and "Y" have the same member "a," even though they have different declaration names. The types are determined by their structures, and in this case, since the structures are the same, they are compatible and valid.

```typescript
type X = {
    a: string;
};
type Y = {
    a: string;
};
const x: X = { a: 'a' };
const y: Y = x; // Valid
```

### TypeScript Fundamental Comparison Rules

The TypeScript comparison process is recursive and executed on types nested at any level.

A type 'X' is compatible with 'Y' if 'Y' has at least the same members as 'X'.

```typescript
type X = {
    a: string;
};
const y = { a: 'A', b: 'B' }; // Valid, as it has at least the same members as X
const r: X = y;
```

Function parameters are compared by types, not by their names:

```typescript
type X = (a: number) => void;
type Y = (a: number) => void;
let x: X = (j: number) => undefined;
let y: Y = (k: number) => undefined;
y = x; // Valid
x = y; // Valid
```

Function return types must be the same:

<!-- skip -->
```typescript
type X = (a: number) => undefined;
type Y = (a: number) => number;
let x: X = (a: number) => undefined;
let y: Y = (a: number) => 1;
y = x; // Invalid
x = y; // Invalid
```

The return type of a source function must be a subtype of the return type of a target function:

<!-- skip -->
```typescript
let x = () => ({ a: 'A' });
let y = () => ({ a: 'A', b: 'B' });
x = y; // Valid
y = x; // Invalid member b is missing
```

Discarding function parameters is allowed, as it is a common practice in JavaScript, for instance using Array.prototype.map():

```typescript
[1, 2, 3].map((element, _index, _array) => element + 'x');
```

Therefore, the following type declarations are completely valid:

```typescript
type X = (a: number) => undefined;
type Y = (a: number, b: number) => undefined;
let x: X = (a: number) => undefined;
let y: Y = (a: number) => undefined; // Missing b parameter
y = x; // Valid
```

Any additional optional parameters of the source type are valid:

```typescript
type X = (a: number, b?: number, c?: number) => undefined;
type Y = (a: number) => undefined;
let x: X = a => undefined;
let y: Y = a => undefined;
y = x; // Valid
x = y; //Valid
```

Any optional parameters of the target type without corresponding parameters in the source type are valid and not an error:

```typescript
type X = (a: number) => undefined;
type Y = (a: number, b?: number) => undefined;
let x: X = a => undefined;
let y: Y = a => undefined;
y = x; // Valid
x = y; // Valid
```

The rest parameter is treated as an infinite series of optional parameters:

```typescript
type X = (a: number, ...rest: number[]) => undefined;
let x: X = a => undefined; //valid
```

Functions with overloads are valid if the overload signature is compatible with its implementation signature:

<!-- skip -->
```typescript
function x(a: string): void;
function x(a: string, b: number): void;
function x(a: string, b?: number): void {
    console.log(a, b);
}
x('a'); // Valid
x('a', 1); // Valid

function y(a: string): void; // Invalid, not compatible with implementation signature
function y(a: string, b: number): void;
function y(a: string, b: number): void {
    console.log(a, b);
}
y('a');
y('a', 1);
```

Function parameter comparison succeeds if the source and target parameters are assignable to supertypes or subtypes (bivariance).

```typescript
// Supertype
class X {
    a: string;
    constructor(value: string) {
        this.a = value;
    }
}
// Subtype
class Y extends X {}
// Subtype
class Z extends X {}

type GetA = (x: X) => string;
const getA: GetA = x => x.a;

// Bivariance does accept supertypes
console.log(getA(new X('x'))); // Valid
console.log(getA(new Y('Y'))); // Valid
console.log(getA(new Z('z'))); // Valid
```

Enums are comparable and valid with numbers and vice versa, but comparing Enum values from different Enum types is invalid.

<!-- skip -->
```typescript
enum X {
    A,
    B,
}
enum Y {
    A,
    B,
    C,
}
const xa: number = X.A; // Valid
const ya: Y = 0; // Valid
X.A === Y.A; // Invalid
```

Instances of a class are subject to a compatibility check for their private and protected members:

<!-- skip -->
```typescript
class X {
    public a: string;
    constructor(value: string) {
        this.a = value;
    }
}

class Y {
    private a: string;
    constructor(value: string) {
        this.a = value;
    }
}

let x: X = new Y('y'); // Invalid
```

The comparison check does not take into consideration the different inheritance hierarchy, for instance:

```typescript
class X {
    public a: string;
    constructor(value: string) {
        this.a = value;
    }
}
class Y extends X {
    public a: string;
    constructor(value: string) {
        super(value);
        this.a = value;
    }
}
class Z {
    public a: string;
    constructor(value: string) {
        this.a = value;
    }
}
let x: X = new X('x');
let y: Y = new Y('y');
let z: Z = new Z('z');
x === y; // Valid
x === z; // Valid even if z is from a different inheritance hierarchy
```

Generics are compared using their structures based on the resulting type after applying the generic parameter, only the final result is compared as a non-generic type.

<!-- skip -->
```typescript
interface X<T> {
    a: T;
}
let x: X<number> = { a: 1 };
let y: X<string> = { a: 'a' };
x === y; // Invalid as the type argument is used in the final structure
```

```typescript
interface X<T> {}
const x: X<number> = 1;
const y: X<string> = 'a';
x === y; // Valid as the type argument is not used in the final structure
```

When generics do not have their type argument specified, all the unspecified arguments are treated as types with "any":

```typescript
type X = <T>(x: T) => T;
type Y = <K>(y: K) => K;
let x: X = x => x;
let y: Y = y => y;
x = y; // Valid
```

Remember:

<!-- skip -->
```typescript
let a: number = 1;
let b: number = 2;
a = b; // Valid, everything is assignable to itself

let c: any;
c = 1; // Valid, all types are assignable to any

let d: unknown;
d = 1; // Valid, all types are assignable to unknown

let e: unknown;
let e1: unknown = e; // Valid, unknown is only assignable to itself and any
let e2: any = e; // Valid
let e3: number = e; // Invalid

let f: never;
f = 1; // Invalid, nothing is assignable to never

let g: void;
let g1: any;
g = 1; // Invalid, void is not assignable to or from anything expect any
g = g1; // Valid
```

Please note that when "strictNullChecks" is enabled, "null" and "undefined" are treated similarly to "void"; otherwise, they are similar to "never".

### Types as sets

In TypeScript, a type is a set of possible values. This set is also referred to as the domain of the type. Each value of a type can be viewed as an element in a set. A type establishes the constraints that every element in the set must satisfy to be considered a member of that set.
The primary task of TypeScript is to check and verify whether one set is a subset of another.

TypeScript supports various types of sets:

| Set term           | TypeScript                      | Notes                                                                                                              |
| ------------------ | ------------------------------- | ------------------------------------------------------------------------------------------------------------------ |
| Empty set          | never                           | “never” contains anything apart itself                                                                             |
| Single element set | undefined / null / literal type |                                                                                                                    |
| Finite set         | boolean / union                 |                                                                                                                    |
| Infinite set       | string / number / object        |                                                                                                                    |
| Universal set      | any / unknown                   | Every element is a member of “any” and every set is a subset of it / “unknown” is a type-safe counterpart of “any” |

Here few examples:

| TypeScript            | Set term               | Example                                                                         |
| --------------------- | ---------------------- | ------------------------------------------------------------------------------- |
| never                 | ∅ (empty set)          | const x: never = 'x'; // Error: Type 'string' is not assignable to type 'never' |
|                       |                        |
| Literal type          | Single element set     | type X = 'X';                                                                   |
|                       |                        | type Y = 7;                                                                     |
|                       |                        |
| Value assignable to T | Value ∈ T (member of)  | type XY = 'X' \| 'Y';                                                           |
|                       |                        | const x: XY = 'X';                                                              |
|                       |                        |
| T1 assignable to T2   | T1 ⊆ T2 (subset of)    | type XY = 'X' \| 'Y';                                                           |
|                       |                        | const x: XY = 'X';                                                              |
|                       |                        | const j: XY = 'J'; // Type '"J"' is not assignable to type 'XY'.                |
|                       |                        |                                                                                 |
| T1 extends T2         | T1 ⊆ T2 (subset of)    | type X = 'X' extends string ? true : false;                                     |
|                       |                        |
| T1 \| T2              | T1 ∪ T2 (union)        | type XY = 'X' \| 'Y';                                                           |
|                       |                        | type JK = 1 \| 2;                                                               |
|                       |                        |
| T1 & T2               | T1 ∩ T2 (intersection) | type X = { a: string }                                                          |
|                       |                        | type Y = { b: string }                                                          |
|                       |                        | type XY = X & Y                                                                 |
|                       |                        | const x: XY = { a: 'a', b: 'b' }                                                |
|                       |                        |
| unknown               | Universal set          | const x: unknown = 1                                                            |

An union, (T1 | T2) creates a wider set (both):

```typescript
type X = {
    a: string;
};
type Y = {
    b: string;
};
type XY = X | Y;
const r: XY = { a: 'a', b: 'x' }; // Valid
```

An intersection, (T1 & T2) create a narrower set (only shared):

<!-- skip -->
```typescript
type X = {
    a: string;
};
type Y = {
    a: string;
    b: string;
};
type XY = X & Y;
const r: XY = { a: 'a' }; // Invalid
const j: XY = { a: 'a', b: 'b' }; // Valid
```

The `extends` keyword could be considered as a “subset of” in this context. It sets a constraint for a type. The extends used with a generic, take the generic as an infinite set and it will constrain it to a more specific type.
Please note that `extends` has nothing to do with hierarchy in a OOP sense (there is no this concept in TypeScript).
TypeScript works with sets and does not have a strict hierarchy, infact, as in the example below, two types could overlap without either being a subtype of the other type (TypeScript considers the structure, shape of the objects).

```typescript
interface X {
    a: string;
}
interface Y extends X {
    b: string;
}
interface Z extends Y {
    c: string;
}
const z: Z = { a: 'a', b: 'b', c: 'c' };
interface X1 {
    a: string;
}
interface Y1 {
    a: string;
    b: string;
}
interface Z1 {
    a: string;
    b: string;
    c: string;
}
const z1: Z1 = { a: 'a', b: 'b', c: 'c' };

const r: Z1 = z; // Valid
```

### Assign a type: Type Declarations and Type Assertions

A type can be assigned in different ways in TypeScript:

#### Type Declaration

In the following example, we use x: X (": Type") to declare a type for the variable x.

```typescript
type X = {
    a: string;
};

const x: X = {
    // Type declaration
    a: 'a',
};
```

If the variable is not in the specified format, TypeScript will report an error. For instance:

<!-- skip -->
```typescript
type X = {
    a: string;
};

const x: X = {
    a: 'a',
    b: 'b', // Error: Object literal may only specify known properties
};
```

#### Type Assertion

It is possible to add an assertion by using the `as` keyword. This tells the compiler that the developer has more information about a type and silences any errors that may occur.

For example:

```typescript
type X = {
    a: string;
};
const x = {
    a: 'a',
    b: 'b',
} as X;
```

In the above example, the object x is asserted to have the type X using the as keyword. This informs the TypeScript compiler that the object conforms to the specified type, even though it has an additional property b not present in the type definition.

Type assertions are useful in situations where a more specific type needs to be specified, especially when working with the DOM. For instance:

```typescript
const myInput = document.getElementById('my_input') as HTMLInputElement;
```

Here, the type assertion as HTMLInputElement is used to tell TypeScript that the result of getElementById should be treated as an HTMLInputElement.
Type assertions can also be used to remap keys, as shown in the example below with template literals:

```typescript
type J<Type> = {
    [Property in keyof Type as `prefix_${string &
        Property}`]: () => Type[Property];
};
type X = {
    a: string;
    b: number;
};
type Y = J<X>;
```

In this example, the type `J<Type>` uses a mapped type with a template literal to remap the keys of Type. It creates new properties with a prefix_ added to each key, and their corresponding values are functions returning the original property values.

It is worth noting that when using a type assertion, TypeScript will not execute excess property checking. Therefore, it is generally preferable to use a Type Declaration when the structure of the object is known in advance.

#### Non-null assertion

This assertion is applied using a post-fix `!` expression operator, which tells TypeScript that a value cannot be null or undefined.

```typescript
let x: null | number;
let y = x!; // number
```

#### Ambient Declarations

Ambient declarations are files that describe types for JavaScript code, they have a file name format as `.d.ts.`. They are usually imported and used to annotate existing JavaScript libraries or to add types to existing JS files in your project.

Many common libraries types can be found at:
<https://github.com/DefinitelyTyped/DefinitelyTyped/>

and can be installed using:

```shell
npm install --save-dev @types/library-name
```

For your defined Ambient Declarations, you can import using the “triple-slash” reference:

<!-- skip -->
```typescript
/// <reference path="./library-types.d.ts" />
```

You can use Ambient Declarations even within JavaScript files using `// @ts-check`.

### Property Checking and Excess Property Checking

TypeScript is based on a structural type system but excess property checking is a property of TypeScript which allows it to check whether an object has the exact properties specified in the type.

Excess Property Checking is performed when assigning object literals to variables or when passing them as arguments to the function's excess property, for instance.

<!-- skip -->
```typescript
type X = {
    a: string;
};
const y = { a: 'a', b: 'b' };
const x: X = y; // Valid because structural typing
const w: X = { a: 'a', b: 'b' }; // Invalid because excess property checking
```

### Weak Types

A type is considered weak when it contains nothing but a set of all-optional properties:

```typescript
type X = {
    a?: string;
    b?: string;
};
```

TypeScript considers an error to assign anything to a weak type when there is no overlap, for instance, the following throws an error:

<!-- skip -->
```typescript
type Options = {
    a?: string;
    b?: string;
};

const fn = (options: Options) => undefined;

fn({ c: 'c' }); // Invalid
```

Although not recommended, if needed, it is possible to bypass this check by using type assertion:

```typescript
type Options = {
    a?: string;
    b?: string;
};
const fn = (options: Options) => undefined;
fn({ c: 'c' } as Options); // Valid
```

Or by adding `unknown` to the index signature to the weak type:

```typescript
type Options = {
    [prop: string]: unknown;
    a?: string;
    b?: string;
};

const fn = (options: Options) => undefined;
fn({ c: 'c' }); // Valid
```

### Strict Object Literal Checking (Freshness)

Strict object literal checking, sometimes referred to as freshness, is a feature in TypeScript that helps catch excess or misspelled properties that would otherwise go unnoticed in normal structural type checks.

When creating an object literal, the TypeScript compiler considers it "fresh." If the object literal is assigned to a variable or passed as a parameter, TypeScript will throw an error if the object literal specifies properties that do not exist in the target type.

However, strict object literal checking does not apply when the type of an object literal is widened, meaning the object literal is structurally type compatible with a broader type.
Here are some examples to illustrate:

<!-- skip -->
```typescript
type X = { a: string };
type Y = { a: string; b: string };

let x: X;
x = { a: 'a', b: 'b' }; // Freshness check: Invalid assignment
var y: Y;
y = { a: 'a', bx: 'bx' }; // Freshness check: Invalid assignment

const fn = (x: X) => console.log(x.a);

fn(x);
fn(y); // No errors, structurally type compatible

fn({ a: 'a', bx: 'b' }); // Freshness check: Invalid argument
```

### Type Inference

TypeScript can infer types when no annotation is provided during:

* Variable initialization.
* Member initialization.
* Setting defaults for parameters.
* Function return type.

For example:

```typescript
let x = 'x'; // The type inferred is string
```

The TypeScript compiler analyzes the value or expression and determines its type based on the available information.

### More advanced inferences

When multiple expressions are used in type inference, TypeScript looks for the "best common types." For instance:

```typescript
let x = [1, 'x', 1, null]; // The type inferred is: (string | number | null)[]
```

If the compiler cannot find the best common types, it returns a union type. For example:

```typescript
let x = [new RegExp('x'), new Date()]; // Type inferred is: (RegExp | Date)[]
```

TypeScript utilizes "contextual typing" based on the variable's location to infer types. In the following example, the compiler knows that `e` is of type `MouseEvent` because of the `click` event type defined in the lib.d.ts file, which contains ambient declarations for various common JavaScript constructs and the DOM:

```typescript
window.addEventListener('click', function (e) {}); // e inferred type is MouseEvent
```

### Type Widening

Type widening is the process in which TypeScript assigns a type to a variable initialized when no type annotation was provided. It allows narrow to wider types but not vice versa.
In the following example:

<!-- skip -->
```typescript
let x = 'x'; // TypeScript infers as string, a wide type
let y: 'y' | 'x' = 'y'; // y types is a union of literal types
y = x; // Invalid Type 'string' is not assignable to type '"x" | "y"'.
```

TypeScript assigns `string` to `x` based on the single value provided during initialization (`x`), this is an example of widening.

TypeScript provides ways to have control of the widening process, for instance using:

### Const

Using the `const` keyword when declaring a variable results in a narrower type inference in TypeScript.

For example:

```typescript
const x = 'x'; // TypeScript infers the type of x as 'x', a narrower type
let y: 'y' | 'x' = 'y';
y = x; // Valid: The type of x is inferred as 'x'
```

By using `const` to declare the variable x, its type is narrowed to the specific literal value 'x'. Since the type of x is narrowed, it can be assigned to the variable y without any error.
The reason the type can be inferred is because `const` variables cannot be reassigned, so their type can be narrowed down to a specific literal type, in this case, the literal type 'x'.

#### const modifier on type parameters

From version 5.0 of TypeScript, it is possible to specify the `const` attribute on a generic type parameter. This allows for inferring the most precise type possible. Let's see an example without using `const`:

```typescript
function identity<T>(value: T) {
    // No const here
    return value;
}
const values = identity({ a: 'a', b: 'b' }); // Type infered is: { a: string; b: string; }
```

As you can see, the properties `a` and `b` are inferred with a type of `string`   .

Now, let's see the difference with the `const` version:

```typescript
function identity<const T>(value: T) {
    // Using const modifier on type parameters
    return value;
}
const values = identity({ a: 'a', b: 'b' }); // Type infered is: { a: "a"; b: "b"; }
```

Now we can see that the properties `a` and `b` are inferred as `const`, so `a` and `b` are treated as string literals rather than just `string` types.

### Explicit Type Annotation

We can be specific and pass a type, in the following example property `x` is of type `number`:

```typescript
const v = {
    x: 1, // Inferred type: number (widening)
};
v.x = 3; // Valid
```

We can make the type annotation more specific by using a union of literal types:

```typescript
const v: { x: 1 | 2 | 3 } = {
    x: 1, // x is now a union of literal types: 1 | 2 | 3
};
v.x = 3; // Valid
```

### Const assertion

Const assertion allows us to be more specific by asserting a `const` type. It can be used on individual properties or on an entire object. Here are a few examples:

On a single property:

```typescript
const v = {
    x: 3 as const,
};
v.x = 3;
```

On an entire object:

```typescript
const v = {
    x: 1,
    y: 2,
} as const;
```

This can be particularly useful when defining the type for a tuple:

```typescript
const x = [1, 2, 3]; // number[]
const y = [1, 2, 3] as const; // Tuple of readonly [1, 2, 3]
```

### Type Narrowing

Type Narrowing is the process in TypeScript where a general type is narrowed down to a more specific type. This occurs when TypeScript analyzes the code and determines that certain conditions or operations can refine the type information.

Narrowing types can occur in different ways, including:

#### Conditions

By using conditional statements, such as `if` or `switch`, TypeScript can narrow down the type based on the outcome of the condition. For example:

```typescript
let x: number | undefined = 10;

if (x !== undefined) {
    x += 100; // The type is number, which had been narrowed by the condition
}
```

#### Throwing or returning

Throwing an error or returning early from a branch can be used to help TypeScript narrow down a type. For example:

```typescript
let x: number | undefined = 10;

if (x === undefined) {
    throw 'error';
}
x += 100;
```

Other ways to narrow down types in TypeScript include:

* `instanceof` operator: Used to check if an object is an instance of a specific class.
* `in` operator: Used to check if a property exists in an object.
* `typeof` operator: Used to check the type of a value at runtime.
* Built-in functions like `Array.isArray()`: Used to check if a value is an array.

#### Discriminated union

Using a "discriminated union" is a pattern in TypeScript where an explicit "tag" is added to objects to distinguish between different types within a union. This pattern is also referred to as a "tagged union." In the following example, the "tag" is represented by the property "type":

```typescript
type A = { type: 'type_a'; value: number };
type B = { type: 'type_b'; value: string };

const x = (input: A | B): string | number => {
    switch (input.type) {
        case 'type_a':
            return input.value + 100; // type is A
        case 'type_b':
            return input.value + 'extra'; // type is B
    }
};
```

#### User-defined type guards

In cases where TypeScript is unable to determine a type, it is possible to write a helper function known as a "user-defined type guard." In the following example, we will utilize a Type Predicate to narrow down the type after applying certain filtering:

```typescript
const data = ['a', null, 'c', 'd', null, 'f'];

const r1 = data.filter(x => x != null); // The type is (string | null)[], TypeScript was not able to infer the type properly

const isValid = (item: string | null): item is string => item !== null; // Custom type guard

const r2 = data.filter(isValid); // The type is fine now string[], by using the predicate type guard we were able to narrow the type
```

## Primitive Types

TypeScript supports 7 primitive types. A primitive data type refers to a type that is not an object and does not have any methods associated with it. In TypeScript, all primitive types are immutable, meaning their values cannot be changed once they are assigned.

### string

The `string` primitive type stores textual data, and the value is always double or single-quoted.

```typescript
const x: string = 'x';
const y: string = 'y';
```

Strings can span multiple lines if surrounded by the backtick (`) character:

```typescript
let sentence: string = `xxx,
   yyy`;
```

### boolean

The `boolean` data type in TypeScript stores a binary value, either `true` or `false`.

```typescript
const isReady: boolean = true;
```

### number

A `number` data type in TypeScript is represented with a 64-bit floating point value. A `number` type can represent integers and fractions.
TypeScript also supports hexadecimal, binary, and octal, for instance:

```typescript
const decimal: number = 10;
const hexadecimal: number = 0xa00d; // Hexadecimal starts with 0x
const binary: number = 0b1010; // Binary starts with 0b
const octal: number = 0o633; // Octal starts with 0c
```

### bigInt

A `bigInt` represents numeric values that are very large (253 – 1) and cannot be represented with a `number`.

A `bigInt` can be created by calling the built-in function `BigInt()` or by adding `n` to the end of any integer numeric literal:

```typescript
const x: bigint = BigInt(9007199254740991);
const y: bigint = 9007199254740991n;
```

Notes: `bigInt` values cannot be mixed with `number` and cannot be used with built-in `Math`, they must be coerced to the same type.

### symbol

JavaScript has a primitive function, Symbol(), that creates a globally unique reference.

```typescript
let sym = Symbol('x'); // Type symbol
```

### null and undefined

`null` and `undefined` types both represent no value or the absence of any value.

The `undefined` type means the value is not assigned or initialized or indicates an unintentional absence of value.

The `null` type means that we know that the field does not have a value, so value is unavailable, it indicates an intentional absence of value.

### Array

An `array` is a data type that can store multiple values of the same type or not. It can be defined using the following syntax:

```typescript
const x: string[] = ['a', 'b'];
const y: Array<string> = ['a', 'b'];
const j: Array<string | number> = ['a', 1, 'b', 2]; // Union
```

TypeScript supports readonly arrays using the following syntax:

<!-- skip -->
```typescript
const x: readonly string[] = ['a', 'b']; // Readonly modifier
const y: ReadonlyArray<string> = ['a', 'b'];
const j: ReadonlyArray<string | number> = ['a', 1, 'b', 2];
j.push('x'); // Invalid
```

TypeScript supports tuple and readonly tuple:

```typescript
const x: [string, number] = ['a', 1];
const y: readonly [string, number] = ['a', 1];
```

### any

The `any` data type represents literally “any” value, it is the default value when TypeScript cannot infer the type or is not specified.

When using `any` TypeScript compiler skips the type checking so there is no type safety when `any` is being used. Generally do not use `any` to silence the compiler when an error occurs, instead focus on fixing the error as with using `any`  it is possible to break contracts and we lose the benefits of TypeScript autocomplete.

The `any` type could be useful during a gradual migration from JavaScript to TypeScript, as it can silence the compiler.

For new projects use TypeScript configuration `noImplicitAny` which enables TypeScript to issue errors where `any` is used or inferred.

The `any`type  is usually a source of errors which can mask real problems with your types. Avoid using it as much as possible.

## Type Annotations

On variables declared using `var`, `let` and `const`, it is possible to optionally add a type:

```typescript
const x: number = 1;
```

TypeScript does a good job of inferring types, especially when simple one, so these declarations in most cases are not necessary.

On functions is possible to add type annotations to parameters:

```typescript
function sum(a: number, b: number) {
    return a + b;
}
```

The following is an example using a anonymous functions (so called lambda function):

```typescript
const sum = (a: number, b: number) => a + b;
```

These annotation can be avoided when a default value for a parameter is present:

```typescript
const sum = (a = 10, b: number) => a + b;
```

Return type annotations can be added to functions:

```typescript
const sum = (a = 10, b: number): number => a + b;
```

This is useful especially for  more complex functions as writing expliciting the return type before an implementation can help better think about the function.

Generally consider annotating type signatures but not the body local variables and add types always to object literals.

## Optional Properties

An object can specify Optional Properties by adding a question mark `?` to the end of the property name:

```typescript
type X = {
    a: number;
    b?: number; // Optional
};
```

It is possible to specify a default value when a property is optional”

```typescript
type X = {
    a: number;
    b?: number;
};
const x = ({ a, b = 100 }: X) => a + b;
```

## Readonly Properties

Is it possible to prevent writing on a property by using the modifier `readonly`which  makes sure that the property cannot be re-written but does not provide any guarantee of total immutability:

```typescript
interface Y {
    readonly a: number;
}

type X = {
    readonly a: number;
};

type J = Readonly<{
    a: number;
}>;

type K = {
    readonly [index: number]: string;
};
```

## Index Signatures

In TypeScript we can use as index signature `string`, `number`, and `symbol`:

```typescript
type K = {
    [name: string | number]: string;
};
const k: K = { x: 'x', 1: 'b' };
console.log(k['x']);
console.log(k[1]);
console.log(k['1']); // same result as k[1]
```

Please note that JavaScript automatically converts an index with `number` to an index with `string` so `k[1]` or k["1"] return the same value.

## Extending Types

It is possible to extend an `interface` (copy members from another type):

```typescript
interface X {
    a: string;
}
interface Y extends X {
    b: string;
}
```

It is also possible to extend from multiple types:

```typescript
interface A {
    a: string;
}
interface B {
    b: string;
}
interface Y extends A, B {
    y: string;
}
```

The `extends` keyword works only on interfaces and classes, for types use an intersection:

```typescript
type A = {
    a: number;
};
type B = {
    b: number;
};
type C = A & B;
```

It is possible to extend a type using an inference but not vice versa:

```typescript
type A = {
    a: string;
};
interface B extends A {
    b: string;
}
```

## Literal Types

A Literal Type is a single element set from a collective type, it defines a very exact value that is a JavaScript primitive.

Literal Types in TypeScript are numbers, strings, and booleans.

Example of literals:

```typescript
const a = 'a'; // string literal type
const b = 1; // numeric literal type
const c = true; // boolean literal type
```

String, Numeric, and Boolean Literal Types are used in the union, type guard, and type aliases.
In the following example you can see a type alias union, `O` can be the only value specified and not any other string:

```typescript
type O = 'a' | 'b' | 'c';
```

## Literal Inference

Literal Inference is a feature in TypeScript that allows the type of a variable or parameter to be inferred based on its value.

In the following example we can see that TypeScript considers `x` a literal type as the value cannot be changed any time later, when instead `y` is inferred as string as it can be modified any time later.

```typescript
const x = 'x'; // literal type of x, because this value cannot be changed
let y = 'y'; // string, as we can change this value
```

In the following example we can see that `o.x` was inferred as a `string` (and not a literal of `a`) as TypeScript considers that the value can be changed any time later.

<!-- skip -->
```typescript
type X = 'a' | 'b';

let o = {
    x: 'a', // this is a wider string
};

const fn = (x: X) => `${x}-foo`;

console.log(fn(o.x)); // Argument of type 'string' is not assignable to parameter of type 'X'
```

As you can see the code throws an error when passing `o.x` to `fn` as X is a narrower type.

We can solve this issue by using type assertion using `const` or the `X` type:

<!-- skip -->
```typescript
let o = {
    x: 'a' as const,
};
```

or:

<!-- skip -->
```typescript
let o = {
    x: 'a' as X,
};
```

## strictNullChecks

`strictNullChecks` is a TypeScript compiler option that enforces strict null checking. When this option is enabled, variables and parameters can only be assigned `null` or `undefined` if they have been explicitly declared to be of that type using the union type `null` | `undefined`. If a variable or parameter is not explicitly declared as nullable, TypeScript will generate an error to prevent potential runtime errors.

## Non-null Assertion Operator (Postfix !)

The Non-null Assertion Operator (Postfix !) is a TypeScript feature that allows you to assert that a variable or property is not `null` or `undefined`, even if TypeScript's static type analysis suggests that it might be. With this feature it is possible to remove any explicit checking.

## Enums

In TypeScript, an enum is a set of named constant values.

```typescript
enum Color {
    Red = '#ff0000',
    Green = '#00ff00',
    Blue = '#0000ff',
}
```

Enums can be defined in different ways:

### Numeric enums

In TypeScript, a numeric enum is an enum where each constant is assigned a numeric value, starting from 0 by default.

```typescript
enum Size {
    Small, // value starts from 0
    Medium,
    Large,
}
```

It is possible to specify custom values by explicitly assigning them:

```typescript
enum Size {
    Small = 10,
    Medium,
    Large,
}
console.log(Size.Medium); // 11
```

### String enums

In TypeScript, a string enum is an enum where each constant is assigned a string value.

```typescript
enum Language {
    English = 'EN',
    Spanish = 'ES',
}
```

Note: TypeScript allows the usage of heterogeneous enums where string and numeric members can coexist.

### Constant enums

A constant enum in TypeScript is a special type of enum where all the values are known at compile time and are inlined wherever the enum is used, resulting in more efficient code.

```typescript
const enum Language {
    English = 'EN',
    Spanish = 'ES',
}
console.log(Language.English);
```

Will be compiled into:

```typescript
console.log('EN' /* Language.English */);
```

Notes:
Const enums have hardcoded values, erasing the enum, which can be more efficient in self-contained libraries but is generally not desirable. Also, const enums cannot have computed members.

### Reverse mapping

In TypeScript, reverse mappings in enums refer to the ability to retrieve the enum member name from its value. By default, enum members have forward mappings from name to value, but reverse mappings can be created by explicitly setting values for each member. Reverse mappings are useful when you need to look up an enum member by its value, or when you need to iterate over all the enum members. Note that only numeric enums members will generate reverse mappings, while string enum members *do not* get a reverse mapping generated at all.

The following enum:

```typescript
enum Grade {
    A = 90,
    B = 80,
    C = 70,
    F = 'fail',
}
```

Compiles to:

<!-- skip -->
```javascript
'use strict';
var Grade;
(function (Grade) {
    Grade[(Grade['A'] = 90)] = 'A';
    Grade[(Grade['B'] = 80)] = 'B';
    Grade[(Grade['C'] = 70)] = 'C';
    Grade['F'] = 'fail';
})(Grade || (Grade = {}));
```

Therefore, mapping values to keys works for numeric enum members, but not for string enum members:

<!-- skip -->
```typescript
enum Grade {
    A = 90,
    B = 80,
    C = 70,
    F = 'fail',
}
const myGrade = Grade.A;
console.log(Grade[myGrade]); // A
console.log(Grade[90]); // A

const failGrade = Grade.F;
console.log(failGrade); // fail
console.log(Grade[failGrade]); // Element implicitly has an 'any' type because index expression is not of type 'number'.
```

### Ambient enums

An ambient enum in TypeScript is a type of enum that is defined in a declaration file (*.d.ts) without an associated implementation. It allows you to define a set of named constants that can be used in a type-safe way across different files without having to import the implementation details in each file.

### Computed and constant members

In TypeScript, a computed member is a member of an enum that has a value calculated at runtime, while a constant member is a member whose value is set at compile-time and cannot be changed during runtime. Computed members are allowed in regular enums, while constant members are allowed in both regular and const enums.

```typescript
// constant members
enum Color {
    Red = 1,
    Green = 5,
    Blue = Red + Green,
}
console.log(Color.Blue); // 6 generation at compilation time
```

```typescript
// computed members
enum Color {
    Red = 1,
    Green = Math.pow(2, 2),
    Blue = Math.floor(Math.random() * 3) + 1,
}
console.log(Color.Blue); // random number generated at run time
```

Enums are denoted by unions comprising their member types. The values of each member can be determined through constant or non-constant expressions, with members possessing constant values being assigned literal types. To illustrate, consider the declaration of type E and its subtypes E.A, E.B, and E.C. In this case, E represents the union E.A | E.B | E.C.

```typescript
const identity = (value: number) => value;

enum E {
    A = 2 * 5, // Numeric literal
    B = 'bar', // String literal
    C = identity(42), // Opaque computed
}

console.log(E.C); //42
```

## Narrowing

TypeScript narrowing is the process of refining the type of a variable within a conditional block. This is useful when working with union types, where a variable can have more than one type.

TypeScript recognizes several ways to narrow the type:

### typeof type guards

The typeof type guard is one specific type guard in TypeScript that checks the type of a variable based on its built-in JavaScript type.

```typescript
const fn = (x: number | string): number => {
    if (typeof x === 'number') {
        return x + 1; // x is number
    }
    return -1;
};
```

### Truthiness narrowing

Truthiness narrowing in TypeScript works by checking whether a variable is truthy or falsy to narrow its type accordingly.

```typescript
const printName = (name: string | null | undefined) => {
    if (name) {
        console.log(name.toUpperCase());
    } else {
        console.log('No name specified');
    }
};
```

### Equality narrowing

Equality narrowing in TypeScript works by checking whether a variable is equal to a specific value or not, to narrow its type accordingly.

```typescript
const logMessage = (status: 'success' | 'error') => {
    switch (status) {
        case 'success':
            console.log('Operation was successful!');
            break;
        case 'error':
            console.log('An error occurred.');
            break;
    }
};
```

### In operator narrowing

The in operator narrowing in TypeScript is a way to narrow the type of a variable based on whether a property exists within the variable's type.

```typescript
type Dog = {
    name: string;
    breed: string;
};

type Cat = {
    name: string;
    likesCream: boolean;
};

const printPet = (pet: Dog | Cat) => {
    if ('breed' in pet) {
        console.log(`This is a ${pet.breed} dog named ${pet.name}.`);
    } else {
        console.log(
            `This is a cat named ${pet.name} that ${
                pet.likesCream ? 'likes' : "doesn't like"
            } cream.`
        );
    }
};
```

### instanceof narrowing

The instanceof operator narrowing in TypeScript is a way to narrow the type of a variable based on its constructor function, by checking if an object is an instance of a certain class or interface.

```typescript
class Square {
    constructor(public width: number) {}
}
class Rectangle {
    constructor(public width: number, public height: number) {}
}
function area(shape: Square | Rectangle) {
    if (shape instanceof Square) {
        return shape.width * shape.width;
    } else {
        return shape.width * shape.height;
    }
}
const square = new Square(5);
const rectangle = new Rectangle(5, 10);
console.log(area(square)); // 25
console.log(area(rectangle)); // 50
```

## Assignments

TypeScript narrowing using assignments is a way to narrow the type of a variable based on the value assigned to it. When a variable is assigned a value, TypeScript infers its type based on the assigned value, and it narrows the type of the variable to match the inferred type.

```typescript
let value: string | number;
value = 'hello';
if (typeof value === 'string') {
    console.log(value.toUpperCase());
}
value = 42;
if (typeof value === 'number') {
    console.log(value.toFixed(2));
}
```

## Control flow analysis

Control flow analysis in TypeScript is a way to statically analyze the code flow to infer the types of variables, allowing the compiler to narrow the types of those variables as needed, based on the results of the analysis.

Prior to TypeScript 4.4, code flow analysis would only be applied to code within an if statement, but from TypeScript 4.4, it can also be applied to conditional expressions and discriminant property accesses indirectly referenced through const variables.

For example:

```typescript
const f1 = (x: unknown) => {
    const isString = typeof x === 'string';
    if (isString) {
        x.length;
    }
};

const f2 = (
    obj: { kind: 'foo'; foo: string } | { kind: 'bar'; bar: number }
) => {
    const isFoo = obj.kind === 'foo';
    if (isFoo) {
        obj.foo;
    } else {
        obj.bar;
    }
};
```

Some examples where narrowing does not occur:

<!-- skip -->
```typescript
const f1 = (x: unknown) => {
    let isString = typeof x === 'string';
    if (isString) {
        x.length; // error, no narrowing because isString it is not const
    }
};

const f6 = (
    obj: { kind: 'foo'; foo: string } | { kind: 'bar'; bar: number }
) => {
    const isFoo = obj.kind === 'foo';
    obj = obj;
    if (isFoo) {
        obj.foo; // Error, no narrowing because obj is assigned in function body
    }
};
```

Notes: Up to five levels of indirection are analyzed in conditional expressions.

## type predicates

Type predicates in TypeScript are functions that return a boolean value and are used to narrow the type of a variable to a more specific type.

```typescript
const isString = (value: unknown): value is string => typeof value === 'string';

const foo = (bar: unknown) => {
    if (isString(bar)) {
        console.log(bar.toUpperCase());
    } else {
        console.log('not a string');
    }
};
```

## Discriminated unions

Discriminated unions in TypeScript are a type of union type that uses a common property, known as the discriminant, to narrow down the set of possible types for the union.

```typescript
type Square = {
    kind: 'square'; // Discriminant
    size: number;
};

type Circle = {
    kind: 'circle'; // Discriminant
    radius: number;
};

type Shape = Square | Circle;

const area = (shape: Shape) => {
    switch (shape.kind) {
        case 'square':
            return Math.pow(shape.size, 2);
        case 'circle':
            return Math.PI * Math.pow(shape.radius, 2);
    }
};

const square: Square = { kind: 'square', size: 5 };
const circle: Circle = { kind: 'circle', radius: 2 };

console.log(area(square)); // 25
console.log(area(circle)); // 12.566370614359172
```

## The never type

When a variable is narrowed to a type that cannot contain any values, the TypeScript compiler will infer that the variable must be of the `never` type. This is because the never type represents a value that can never be produced.

```typescript
const printValue = (val: string | number) => {
    if (typeof val === 'string') {
        console.log(val.toUpperCase());
    } else if (typeof val === 'number') {
        console.log(val.toFixed(2));
    } else {
        // val has type never here because it can never be anything other than a string or a number
        const neverVal: never = val;
        console.log(`Unexpected value: ${neverVal}`);
    }
};
```

## Exhaustiveness checking

Exhaustiveness checking is a feature in TypeScript that ensures all possible cases of a discriminated union are handled in a switch statement or an if statement.

```typescript
type Direction = 'up' | 'down';

const move = (direction: Direction) => {
    switch (direction) {
        case 'up':
            console.log('Moving up');
            break;
        case 'down':
            console.log('Moving down');
            break;
        default:
            const exhaustiveCheck: never = direction;
            console.log(exhaustiveCheck); // this line will never be executed
    }
};
```

The `never` type is used to ensure that the default case is exhaustive and that TypeScript will raise an error if a new value is added to the Direction type without being handled in the switch statement.

## Object Types

In TypeScript, object types describe the shape of an object. They specify the names and types of the object's properties, as well as whether those properties are required or optional.

In TypeScript, you can define object types in two primary ways:

Interface which defines the shape of an object by specifying the names, types, and optionality of its properties.

```typescript
interface User {
    name: string;
    age: number;
    email?: string;
}
```

Type alias, similar to an interface, defines the shape of an object. However, it can also create a new custom type that is based on an existing type or a combination of existing types. This includes defining union types, intersection types, and other complex types.

```typescript
type Point = {
    x: number;
    y: number;
};
```

It also possible to define a type anonymously:

```typescript
const sum = (x: { a: number; b: number }) => x.a + x.b;
console.log(sum({ a: 5, b: 1 }));
```

## Tuple Type

A Tuple Type is a type that represents an array with a fixed number of elements and their corresponding types. A tuple type enforces a specific number of elements and their respective types in a fixed order. Tuple types are useful when you want to represent a collection of values with specific types, where the position of each element in the array has a specific meaning.

```typescript
type Point = [number, number];
```

## Fixed length tuple

A Fixed length tuple is a specific type of tuple that enforces a fixed number of elements of specific types, and disallows any modifications to the length of the tuple once it is defined.

Fixed length tuples are useful when you need to represent a collection of values with a specific number of elements and specific types, and you want to ensure that the length and types of the tuple cannot be changed inadvertently.

<!-- skip -->
```typescript
const x = [10, 'hello'] as const;
x.push(2); // Error
```

## Union Type

A Union Type is a type that represents a value that can be one of several types. Union Types are denoted using the | symbol between each possible type.

```typescript
let x: string | number;
x = 'hello'; // Valid
x = 123; // Valid
```

## Intersection Types

An Intersection Type is a type that represents a value that has all the properties of two or more types. Intersection Types are denoted using the `&`` symbol between each type.

```typescript
type X = {
    a: string;
};

type Y = {
    b: string;
};

type J = X & Y; // Intersection

const j: J = {
    a: 'a',
    b: 'b',
};
```

## Type Indexing

Type indexing refers to the ability to define types that can be indexed by a key that is not known in advance, using an index signature to specify the type for properties that are not explicitly declared.

```typescript
type Dictionary<T> = {
    [key: string]: T;
};
const myDict: Dictionary<string> = { a: 'a', b: 'b' };
console.log(myDict['a']); // return a
```

## Type from Value

"Type from Value" in TypeScript refers to the automatic inference of a type from a value or expression through type inference.

```typescript
const x = 'x'; // TypeScript can automatically infer that the type of the message variable is string
```

## Type from Func Return

Type from Func Return refers to the ability to automatically infer the return type of a function based on its implementation. This allows TypeScript to determine the type of the value returned by the function without explicit type annotations.

```typescript
const add = (x: number, y: number) => x + y; // TypeScript can infer that the return type of the function is a number
```

## Type from Module

Type from Module refers to the ability to use a module's exported values to automatically infer their types. When a module exports a value with a specific type, TypeScript can use that information to automatically infer the type of that value when it is imported into another module.

<!-- skip -->
```typescript
// calc.ts
export const add = (x: number, y: number) => x + y;
// index.ts
import { add } from 'calc';
const r = add(1, 2); // r is number
```

## Mapped types

Mapped types in TypeScript allow you to create new types based on an existing type by transforming each property using a mapping function. By mapping existing types, you can create new types that represent the same information in a different format. To create a mapped type, you access the properties of an existing type using the `keyof` operator and then alter them to produce a new type.
In the following example:

```typescript
type MyMappedType<T> = {
    [P in keyof T]: T[P][];
};
type MyType = {
    foo: string;
    bar: number;
};
type MyNewType = MyMappedType<MyType>;
const x: MyNewType = {
    foo: ['hello', 'world'],
    bar: [1, 2, 3],
};
```

we define MyMappedType to map over T's properties, creating a new type with each property as an array of its original type. Using this, we create MyNewType to represent the same info as MyType, but with each property as an array.

## Conditional Types

Conditional types are a way to create a type that depends on a condition, where the type to be created is determined based on the result of the condition. They are defined using the `extends` keyword and a ternary operator to conditionally choose between two types.

```typescript
type IsArray<T> = T extends any[] ? true : false;

const myArray = [1, 2, 3];
const myNumber = 42;

type IsMyArrayAnArray = IsArray<typeof myArray>; // Type true
type IsMyNumberAnArray = IsArray<typeof myNumber>; // Type false
```

## Distributive conditional types

Distributive conditional types are a feature that allow a type to be distributed over a union of types, by applying a transformation to each member of the union individually.
This can be especially useful when working with mapped types or higher-order types.

```typescript
type Nullable<T> = T extends any ? T | null : never;
type NumberOrBool = number | boolean;
type NullableNumberOrBool = Nullable<NumberOrBool>; // number | boolean | null
```

## “infer” Type inference in conditional types

The `infer`keyword is used in conditional types to infer (extract) the type of a generic parameter from a type that depends on it. This allows you to write more flexible and reusable type definitions.

```typescript
type ElementType<T> = T extends (infer U)[] ? U : never;
type Numbers = ElementType<number[]>; // number
type Strings = ElementType<string[]>; // string
```

## Predefined conditional types

In TypeScript, predefined conditional types are built-in conditional types provided by the language. They are designed to perform common type transformations based on the characteristics of a given type.

`Exclude<UnionType, ExcludedType>`: This type removes all the types from Type that are assignable to ExcludedType.

`Extract<Type, Union>`: This type extracts all the types from Union that are assignable to Type.

`NonNullable<Type>`: This type removes null and undefined from Type.

`ReturnType<Type>`: This type extracts the return type of a function Type.

`Parameters<Type>`: This type extracts the parameter types of a function Type.

`Required<Type>`: This type makes all properties in Type required.

`Partial<Type>`: This type makes all properties in Type optional.

`Readonly<Type>`: This type makes all properties in Type readonly.

## Template Union Types

Template union types can be used to merge and manipulate text inside the type system for instance:

```typescript
type Status = 'active' | 'inactive';
type Products = 'p1' | 'p2';
type ProductId = `id-${Products}-${Status}`; // "id-p1-active" | "id-p1-inactive" | "id-p2-active" | "id-p2-inactive"
```

## Any type

The `any` type is a special type (universal supertype) that can be used to represent any type of value (primitives, objects, arrays, functions, errors, symbols). It is often used in situations where the type of a value is not known at compile time, or when working with values from external APIs or libraries that do not have TypeScript typings.

By utilizing any type, you are indicating to the TypeScript compiler that values should be represented without any limitations. In order to maximizing type safety in your code consider the following:

* Limit the usage of any to specific cases where the type is truly unknown.
* Do not return `any` types from a function as you will lose type safety in the code using that function weakening your type safety.
* Instead of `any` use @ts-ignore` if you need to silence the compiler.

```typescript
let value: any;
value = true; // Valid
value = 7; // Valid
```

## Unknown type

In TypeScript, the unknown type represents a value that is of an unknown type. Unlike `any` type, which allows for any type of value, unknown requires a type check or assertion before it can be used in a specific way so no operations are permitted on an `unknown` without first asserting or narrowing to a more specific type.

The `unknown` type is only assignable to any type and the unknown type itself, it is a type-safe alternative to any.

<!-- skip -->
```typescript
let value: unknown;

let value1: unknown = value; // Valid
let value2: any = value; // Valid
let value3: boolean = value; // Invalid
let value4: number = value; // Invalid
```

```typescript
const add = (a: unknown, b: unknown): number | undefined =>
    typeof a === 'number' && typeof b === 'number' ? a + b : undefined;
console.log(add(1, 2)); // 3
console.log(add('x', 2)); // undefined
```

## Void type

The `void` type is used to indicate that a function does not return a value.

```typescript
const sayHello = (): void => {
    console.log('Hello!');
};
```

## Never type

The `never` type represents values that never occur. It is used to denote functions or expressions that never return or throw an error.

For instance an infinite loop:

```typescript
const infiniteLoop = (): never => {
    while (true) {
        // do something
    }
};
```

Throwing an error:

```typescript
const throwError = (message: string): never => {
    throw new Error(message);
};
```

The `never` type is useful in ensuring type safety and catching potential errors in your code. It helps TypeScript analyze and infer more precise types when used in combination with other types and control flow statements, for instance:

```typescript
type Direction = 'up' | 'down';
const move = (direction: Direction): void => {
    switch (direction) {
        case 'up':
            // move up
            break;
        case 'down':
            // move down
            break;
        default:
            const exhaustiveCheck: never = direction;
            throw new Error(`Unhandled direction: ${exhaustiveCheck}`);
    }
};
```

## Interface and Type

### Common Syntax

In TypeScript, interfaces define the structure of objects, specifying the names and types of properties or methods that an object must have. The common syntax for defining an interface in TypeScript is as follows:

<!-- skip -->
```typescript
interface InterfaceName {
    property1: Type1;
    // ...
    method1(arg1: ArgType1, arg2: ArgType2): ReturnType;
    // ...
}
```

Similarly for type definition:

<!-- skip -->
```typescript
type TypeName = {
    property1: Type1;
    // …
    method1(arg1: ArgType1, arg2: ArgType2): ReturnType;
    // ...
};
```

`interface InterfaceName` or `type TypeName`: Defines the name of the interface.
`property1`: `Type1`: Specifies the properties of the interface along with their corresponding types. Multiple properties can be defined, each separated by a semicolon.
`method1(arg1: ArgType1, arg2: ArgType2): ReturnType;` method2(): void;: Specifies the methods of the interface. Methods are defined with their names, followed by a parameter list in parentheses and the return type. Multiple methods can be defined, each separated by a semicolon.

Example interface:

```typescript
interface Person {
    name: string;
    age: number;
    greet(): void;
}
```

Example of type:

```typescript
type TypeName = {
    property1: string;
    method1(arg1: string, arg2: string): string;
};
```

In TypeScript, types are used to define the shape of data and enforce type checking. There are several common syntaxes for defining types in TypeScript, depending on the specific use case. Here are some examples:

### Basic types

```typescript
let myNumber: number = 123; // number type
let myBoolean: boolean = true; // boolean type
let myArray: string[] = ['a', 'b']; // array of strings
let myTuple: [string, number] = ['a', 123]; // tuple
```

### Objects and interfaces

```typescript
const x: { name: string; age: number } = { name: 'Simon', age: 7 };
```

### Union and intersection types

```typescript
type MyType = string | number; // union type
let myUnion: MyType = 'hello'; // can be a string
myUnion = 123; // or a number

type TypeA = { name: string };
type TypeB = { age: number };
type CombinedType = TypeA & TypeB; // intersection type
let myCombined: CombinedType = { name: 'John', age: 25 }; // object with both name and age properties
```

## Built-in Type Primitives

TypeScript has several built-in type primitives that can be used to define variables, function parameters, and return types:

* `number`: Represents numeric values, including integers and floating-point numbers.
* `string`: Represents textual data
* `boolean`: Represents logical values, which can be either true or false.
* `null`: Represents the absence of a value.
* `undefined`: Represents a value that has not been assigned or has not been defined.
* `symbol`: Represents a unique identifier. Symbols are typically used as keys for object properties.
* `bigint`: Represents arbitrary-precision integers.
* `any`: Represents a dynamic or unknown type. Variables of type any can hold values of any type, and they bypass type checking.
* `void`: Represents the absence of any type. It is commonly used as the return type of functions that do not return a value.
* `never`: Represents a type for values that never occur. It is typically used as the return type of functions that throw an error or enter an infinite loop.

## Common Built-in JS Objects

TypeScript is a superset of JavaScript, it includes all the commonly used built-in JavaScript objects. You can find an extensive list of these objects on the Mozilla Developer Network (MDN) documentation website:
<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects>

Here is a list of some commonly used built-in JavaScript objects:

* Function
* Object
* Boolean
* Error
* Number
* BigInt
* Math
* Date
* String
* RegExp
* Array
* Map
* Set
* Promise
* Intl

## Overloads

Function overloads in TypeScript allow you to define multiple function signatures for a single function name, enabling you to define functions that can be called in multiple ways. Here's an example:

```typescript
// Overloads
function sayHi(name: string): string;
function sayHi(names: string[]): string[];

// Implementation
function sayHi(name: unknown): unknown {
    if (typeof name === 'string') {
        return `Hi, ${name}!`;
    } else if (Array.isArray(name)) {
        return name.map(name => `Hi, ${name}!`);
    }
    throw new Error('Invalid value');
}

sayHi('xx'); // Valid
sayHi(['aa', 'bb']); // Valid
```

Here's another example of using function overloads within a `class`:

```typescript
class Greeter {
    message: string;

    constructor(message: string) {
        this.message = message;
    }

    // overload
    sayHi(name: string): string;
    sayHi(names: string[]): ReadonlyArray<string>;

    // implementation
    sayHi(name: unknown): unknown {
        if (typeof name === 'string') {
            return `${this.message}, ${name}!`;
        } else if (Array.isArray(name)) {
            return name.map(name => `${this.message}, ${name}!`);
        }
        throw new Error('value is invalid');
    }
}
console.log(new Greeter('Hello').sayHi('Simon'));
```

## Get & Set

Getters and setters are special methods that allow you to define custom access and modification behavior for class properties. They enable you to encapsulate the internal state of an object and provide additional logic when getting or setting the values of properties.
In TypeScript, getters and setters are defined using the `get` and `set` keywords respectively. Here's an example:

```typescript
class MyClass {
    private _myProperty: string;

    constructor(value: string) {
        this._myProperty = value;
    }
    get myProperty(): string {
        return this._myProperty;
    }
    set myProperty(value: string) {
        this._myProperty = value;
    }
}
```

## Merging and Extension

Merging and extension refer to two different concepts related to working with types and interfaces.

Merging allows you to combine multiple declarations of the same name into a single definition, for example, when you define an interface with the same name multiple times:

```typescript
interface X {
    a: string;
}

interface X {
    b: number;
}

const person: X = {
    a: 'a',
    b: 7,
};
```

Extension refers to the ability to extend or inherit from existing types or interfaces to create new ones. It is a mechanism to add additional properties or methods to an existing type without modifying its original definition. Example:

```typescript
interface Animal {
    name: string;
    eat(): void;
}

interface Bird extends Animal {
    sing(): void;
}

const dog: Bird = {
    name: 'Bird 1',
    eat() {
        console.log('Eating');
    },
    sing() {
        console.log('Singing');
    },
};
```

## Differences between Type and Interface

Declaration merging (augmentation): Interfaces support declaration merging, which means that you can define multiple interfaces with the same name, and TypeScript will merge them into a single interface with the combined properties and methods. On the other hand, types do not support declaration merging. This can be helpful when you want to add extra functionality or customize existing types without modifying the original definitions or patching missing or incorrect types.

```typescript
interface A {
    x: string;
}
interface A {
    y: string;
}
const j: A = {
    x: 'xx',
    y: 'yy',
};
```

Extending other types/interfaces: Both types and interfaces can extend other types/interfaces, but the syntax is different. With interfaces, you use the `extends` keyword to inherit properties and methods from other interfaces. However, an interface cannot extend a complex type like a union type.

```typescript
interface A {
    x: string;
    y: number;
}
interface B extends A {
    z: string;
}
const car: B = {
    x: 'x',
    y: 123,
    z: 'z',
};
```

For types, you use the & operator to combine multiple types into a single type (intersection).

```typescript
interface A {
    x: string;
    y: number;
}

type B = A & {
    j: string;
};

const c: B = {
    x: 'x',
    y: 123,
    j: 'j',
};
```

Union and intersection types: Types are more flexible when it comes to defining union and intersection types. With the `type` keyword, you can easily create union types using the `|` operator and intersection types using the `&` operator. While interfaces can also represent union types indirectly, they don't have built-in support for intersection types.

```typescript
type Department = 'dep-x' | 'dep-y'; // Union

type Person = {
    name: string;
    age: number;
};

type Employee = {
    id: number;
    department: Department;
};

type EmployeeInfo = Person & Employee; // Intersection
```

Example with interfaces:

```typescript
interface A {
    x: 'x';
}
interface B {
    y: 'y';
}

type C = A | B; // Union of interfaces
```

## Class

### Class Common Syntax

The `class` keyword is used in TypeScript to define a class. Below, you can see an example:

```typescript
class Person {
    private name: string;
    private age: number;
    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }
    public sayHi(): void {
        console.log(
            `Hello, my name is ${this.name} and I am ${this.age} years old.`
        );
    }
}
```

The `class` keyword is used to define a class named "Person".

The class has two private properties: name of type `string` and age of type `number`.

The constructor is defined using the `constructor` keyword. It takes name and age as parameters and assigns them to the corresponding properties.

The class has a `public` method named sayHi that logs a greeting message.

To create an instance of a class in TypeScript, you can use the `new` keyword followed by the class name, followed by parentheses (). For instance:

<!-- skip -->
```typescript
const myObject = new Person('John Doe', 25);
myObject.sayHi(); // output: Hello, my name is John Doe and I am 25 years old.
```

### Constructor

Constructors are special methods within a class that are used to initialize the object's properties when an instance of the class is created.

```typescript
class Person {
    public name: string;
    public age: number;

    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }

    sayHello() {
        console.log(
            `Hello, my name is ${this.name} and I'm ${this.age} years old.`
        );
    }
}

const john = new Person('Simon', 17);
john.sayHello();
```

It is possible to overload a constructor using the following syntax:

```typescript
type Sex = 'm' | 'f';

class Person {
    name: string;
    age: number;
    sex: Sex;

    constructor(name: string, age: number, sex?: Sex);
    constructor(name: string, age: number, sex: Sex) {
        this.name = name;
        this.age = age;
        this.sex = sex ?? 'm';
    }
}

const p1 = new Person('Simon', 17);
const p2 = new Person('Alice', 22, 'f');
```

In TypeScript, it is possible to define multiple constructor overloads, but you can have only one implementation that must be compatible with all the overloads, this can be achieved  by using an optional parameter.

```typescript
class Person {
    name: string;
    age: number;

    constructor();
    constructor(name: string);
    constructor(name: string, age: number);
    constructor(name?: string, age?: number) {
        this.name = name ?? 'Unknown';
        this.age = age ?? 0;
    }

    displayInfo() {
        console.log(`Name: ${this.name}, Age: ${this.age}`);
    }
}

const person1 = new Person();
person1.displayInfo(); // Name: Unknown, Age: 0

const person2 = new Person('John');
person2.displayInfo(); // Name: John, Age: 0

const person3 = new Person('Jane', 25);
person3.displayInfo(); // Name: Jane, Age: 25
```

### Private and Protected Constructors

In TypeScript, constructors can be marked as private or protected, which restricts their accessibility and usage.

Private Constructors:
Can be called only within the class itself. Private constructors are often used in scenarios where you want to enforce a singleton pattern or restrict the creation of instances to a factory method within the class

Protected Constructors:
Protected constructors are useful when you want to create a base class that should not be instantiated directly but can be extended by subclasses.

```typescript
class BaseClass {
    protected constructor() {}
}

class DerivedClass extends BaseClass {
    private value: number;

    constructor(value: number) {
        super();
        this.value = value;
    }
}

// attempting to instantiate the base class directly will result in an error
// const baseObj = new BaseClass(); // Error: Constructor of class 'BaseClass' is protected.

// create an instance of the derived class
const derivedObj = new DerivedClass(10);
```

### Access modifiers

Access modifiers `private`, `protected`, and `public` are used to control the visibility and accessibility of class members, such as properties and methods, in TypeScript classes. These modifiers are essential for enforcing encapsulation and establishing boundaries for accessing and modifying the internal state of a class.

The `private` modifier restricts access to the class member only within the containing class.

The `protected` modifier allows access to the class member within the containing class and its derived classes.

The `public` modifier provides unrestricted access to the class member, allowing it to be accessed from anywhere."

### Auto-Accessors in Classes

TypeScript version 4.9 adds support for auto-accessors, a forthcoming ECMAScript feature. They resemble class properties but are declared with the "accessor" keyword.

```typescript
class Animal {
    accessor name: string;

    constructor(name: string) {
        this.name = name;
    }
}
```

Auto-accessors are "de-sugared" into private `get` and `set` accessors, operating on an inaccessible property.

<!-- skip -->
```typescript
class Animal {
    #__name: string;

    get name() {
        return this.#__name;
    }
    set name(value: string) {
        this.#__name = name;
    }

    constructor(name: string) {
        this.name = name;
    }
}
```

### this

In TypeScript, the `this` keyword refers to the current instance of a class within its methods or constructors. It allows you to access and modify the properties and methods of the class from within its own scope.
It provides a way to access and manipulate the internal state of an object within its own methods.

```typescript
class Person {
    private name: string;
    constructor(name: string) {
        this.name = name;
    }
    public introduce(): void {
        console.log(`Hello, my name is ${this.name}.`);
    }
}

const person1 = new Person('Alice');
person1.introduce(); // Hello, my name is Alice.
```

### Parameter Properties

Parameter properties allow you to declare and initialize class properties directly within the constructor parameters avoiding boilerplate code, example:

```typescript
class Person {
    constructor(private name: string, public age: number) {
        // the "private" and "public" keywords in the constructor
        // automatically declare and initialize the corresponding class properties.
    }
    public introduce(): void {
        console.log(
            `Hello, my name is ${this.name} and I am ${this.age} years old.`
        );
    }
}
const person = new Person('Alice', 25);
person.introduce();
```

### Abstract Classes

Abstract Classes are used in TypeScript mainly for inheritance, they provide a way to define common properties and methods that can be inherited by subclasses.
This is useful when you want to define common behavior and enforce that subclasses implement certain methods. They provide a way to create a hierarchy of classes where the abstract base class provides a shared interface and common functionality for the subclasses.

```typescript
abstract class Animal {
    protected name: string;

    constructor(name: string) {
        this.name = name;
    }

    abstract makeSound(): void;
}

class Cat extends Animal {
    makeSound(): void {
        console.log(`${this.name} meows.`);
    }
}

const cat = new Cat('Whiskers');
cat.makeSound(); // Output: Whiskers meows.
```

### With Generics

Classes with generics allow you to define reusable classes which can work with different types.

```typescript
class Container<T> {
    private item: T;

    constructor(item: T) {
        this.item = item;
    }

    getItem(): T {
        return this.item;
    }

    setItem(item: T): void {
        this.item = item;
    }
}

const container1 = new Container<number>(42);
console.log(container1.getItem()); //  42

const container2 = new Container<string>('Hello');
container2.setItem('World');
console.log(container2.getItem()); // World
```

### Decorators

Decorators provide a mechanism to add metadata, modify behavior, validate, or extend the functionality of the target element. They are functions that execute at runtime. Multiple decorators can be applied to a declaration.

Decorators are experimental features, and the following examples are only compatible with TypeScript version 5 or above using ES6.

For TypeScript versions prior to 5, they should be enabled using the `experimentalDecorators` property in your `tsconfig.json` or by using `--experimentalDecorators` in your command line (but the following example won't work).

Some of the common use cases for decorators include:

* Watching property changes
* Watching method calls
* Adding extra properties or methods
* Runtime validation
* Automatic serialization and deserialization
* Logging
* Authorization and authentication
* Error guarding

Note: Decorators for version 5 do not allow decorating parameters.

Types of decorators:

#### Class decorators

Class decorators are useful for extending an existing class, such as adding properties or methods, or collecting instances of a class. In the following example, we add a `toString` method that converts the class into a string representation.

```typescript
type Constructor<T = {}> = new (...args: any[]) => T;

function toString<Class extends Constructor>(
    Value: Class,
    context: ClassDecoratorContext<Class>
) {
    return class extends Value {
        constructor(...args: any[]) {
            super(...args);
            console.log(JSON.stringify(this));
            console.log(JSON.stringify(context));
        }
    };
}

@toString
class Person {
    name: string;

    constructor(name: string) {
        this.name = name;
    }

    greet() {
        return 'Hello, ' + this.name;
    }
}
const person = new Person('Simon');
/* Logs:
{"name":"Simon"}
{"kind":"class","name":"Person"}
*/
```

#### Property Decorator

Property decorators are useful for modifying the behavior of a property, such as changing the initialization values. In the following code, we have a script that sets a property to always be in uppercase:

```typescript
function upperCase<T>(
    target: undefined,
    context: ClassFieldDecoratorContext<T, string>
) {
    return function (this: T, value: string) {
        return value.toUpperCase();
    };
}

class MyClass {
    @upperCase
    prop1 = 'hello!';
}

console.log(new MyClass().prop1); // Logs: HELLO!
```

#### Method Decorator

Method decorators allow you to change or enhance the behavior of methods. Below is an example of a simple logger:

```typescript
function log<This, Args extends any[], Return>(
    target: (this: This, ...args: Args) => Return,
    context: ClassMethodDecoratorContext<
        This,
        (this: This, ...args: Args) => Return
    >
) {
    const methodName = String(context.name);

    function replacementMethod(this: This, ...args: Args): Return {
        console.log(`LOG: Entering method '${methodName}'.`);
        const result = target.call(this, ...args);
        console.log(`LOG: Exiting method '${methodName}'.`);
        return result;
    }

    return replacementMethod;
}

class MyClass {
    @log
    sayHello() {
        console.log('Hello!');
    }
}

console.log(new MyClass().sayHello()); // Logs: Hello!
```

#### Getter and Setter Decorators

Getter and setter decorators allow you to change or enhance the behavior of class accessors. They are useful, for instance, for validating property assignments. Here's a simple example for a getter decorator:

```typescript
function range<This, Return extends number>(min: number, max: number) {
    return function (
        target: (this: This) => Return,
        context: ClassGetterDecoratorContext<This, Return>
    ) {
        return function (this: This): Return {
            const value = target.call(this);
            if (value < min || value > max) {
                throw 'Invalid';
            }
            Object.defineProperty(this, context.name, {
                value,
                enumerable: true,
            });
            return value;
        };
    };
}

class MyClass {
    private _value = 0;

    constructor(value: number) {
        this._value = value;
    }
    @range(1, 100)
    get getValue(): number {
        return this._value;
    }
}

const obj = new MyClass(10);
console.log(obj.getValue); // Valid: 10

const obj2 = new MyClass(999);
console.log(obj2.getValue); // Throw: Invalid!
```

### Inheritance

Inheritance refers to the mechanism by which a class can inherit properties and methods from another class, known as the base class or superclass. The derived class, also called the child class or subclass, can extend and specialize the functionality of the base class by adding new properties and methods or overriding existing ones.

```typescript
class Animal {
    name: string;

    constructor(name: string) {
        this.name = name;
    }

    speak(): void {
        console.log('The animal makes a sound');
    }
}

class Dog extends Animal {
    breed: string;

    constructor(name: string, breed: string) {
        super(name);
        this.breed = breed;
    }

    speak(): void {
        console.log('Woof! Woof!');
    }
}

// create an instance of the base class
const animal = new Animal('Generic Animal');
animal.speak(); // The animal makes a sound

// create an instance of the derived class
const dog = new Dog('Max', 'Labrador');
dog.speak(); // Woof! Woof!"
```

TypeScript does not support multiple inheritance in the traditional sense and instead allows inheritance from a single base class.
TypeScript supports multiple interfaces. An interface can define a contract for the structure of an object, and a class can implement multiple interfaces. This allows a class to inherit behavior and structure from multiple sources.

```typescript
interface Flyable {
    fly(): void;
}

interface Swimmable {
    swim(): void;
}

class FlyingFish implements Flyable, Swimmable {
    fly() {
        console.log('Flying...');
    }

    swim() {
        console.log('Swimming...');
    }
}

const flyingFish = new FlyingFish();
flyingFish.fly();
flyingFish.swim();
```

The `class` keyword in TypeScript, similar to JavaScript, is often referred to as syntactic sugar. It was introduced in ECMAScript 2015 (ES6) to offer a more familiar syntax for creating and working with objects in a class-based manner. However, it's important to note that TypeScript, being a superset of JavaScript, ultimately compiles down to JavaScript, which remains prototype-based at its core.

### Statics

TypeScript has static members. To access the static members of a class, you can use the class name followed by a dot, without the need to create an object.

```typescript
class OfficeWorker {
    static memberCount: number = 0;

    constructor(private name: string) {
        OfficeWorker.memberCount++;
    }
}

const w1 = new OfficeWorker('James');
const w2 = new OfficeWorker('Simon');
const total = OfficeWorker.memberCount;
console.log(total);
```

### Property initialization

There are several ways how you can initialize properties for a class in TypeScript:

Inline:

In the following example these initial values will be used when an instance of the class is created.

```typescript
class MyClass {
    property1: string = 'default value';
    property2: number = 42;
}
```

In the constructor:

```typescript
class MyClass {
    property1: string;
    property2: number;

    constructor() {
        this.property1 = 'default value';
        this.property2 = 42;
    }
}
```

Using constructor parameters:

```typescript
class MyClass {
    constructor(
        private property1: string = 'default value',
        public property2: number = 42
    ) {
        // there is no need to assign the values to the properties explicitly.
    }
    log() {
        console.log(this.property2);
    }
}
const x = new MyClass();
x.log();
```

### Method overloading

Method overloading allows a class to have multiple methods with the same name but different parameter types or a different number of parameters. This allows us to call a method in different ways based on the arguments passed.

```typescript
class MyClass {
    add(a: number, b: number): number; // Overload signature 1
    add(a: string, b: string): string; // Overload signature 2

    add(a: number | string, b: number | string): number | string {
        if (typeof a === 'number' && typeof b === 'number') {
            return a + b;
        }
        if (typeof a === 'string' && typeof b === 'string') {
            return a.concat(b);
        }
        throw new Error('Invalid arguments');
    }
}

const r = new MyClass();
console.log(r.add(10, 5));
```

## Generics

Generics allow you to create reusable components and functions that can work with multiple types. With generics, you can parameterize types, functions, and interfaces, allowing them to operate on different types without explicitly specifying them beforehand.

Generics allow you to make code more flexible and reusable.

### Generic Type

To define a generic type, you use angle brackets (<>) to specify the type parameters, for instance:

```typescript
function identity<T>(arg: T): T {
    return arg;
}
const a = identity('x');
const b = identity(123);

const getLen = <T>(data: ReadonlyArray<T>) => data.length;
const len = getLen([1, 2, 3]);
```

### Generic Classes

Generics can be applied also to classes, in this way they can work with multiple types by using type parameters. This is useful to create reusable class definitions that can operate on different data types while maintaining type safety.

```typescript
class Container<T> {
    private item: T;

    constructor(item: T) {
        this.item = item;
    }

    getItem(): T {
        return this.item;
    }
}

const numberContainer = new Container<number>(123);
console.log(numberContainer.getItem()); // 123

const stringContainer = new Container<string>('hello');
console.log(stringContainer.getItem()); // hello
```

### Generic Constraints

Generic parameters can be constrained using the `extends` keyword followed by a type or interface that the type parameter must satisfy.

In the following example T it is must containing a properly `length` in order to be valid:

<!-- skip -->
```typescript
const printLen = <T extends { length: number }>(value: T): void => {
    console.log(value.length);
};

printLen('Hello'); // 5
printLen([1, 2, 3]); // 3
printLen({ length: 10 }); // 10
printLen(123); // Invalid
```

An interesting feature of generic introduced in version 3.4 RC is Higher order function type inference which introduced  propagated generic type arguments:

```typescript
declare function pipe<A extends any[], B, C>(
    ab: (...args: A) => B,
    bc: (b: B) => C
): (...args: A) => C;

declare function list<T>(a: T): T[];
declare function box<V>(x: V): { value: V };

const listBox = pipe(list, box); // <T>(a: T) => { value: T[] }
const boxList = pipe(box, list); // <V>(x: V) => { value: V }[]
```

This functionality allows more easily typed safe pointfree style programming which is common in functional programming.

### Generic contextual narrowing

Contextual narrowing for generics is the mechanism in TypeScript that allows the compiler to narrow down the type of a generic parameter based on the context in which it is used, it is useful when working with generic types in conditional statements:

```typescript
function process<T>(value: T): void {
    if (typeof value === 'string') {
        // Value is narrowed down to type 'string'
        console.log(value.length);
    } else if (typeof value === 'number') {
        // Value is narrowed down to type 'number'
        console.log(value.toFixed(2));
    }
}

process('hello'); // 5
process(3.14159); // 3.14
```

## Erased Structural Types

In TypeScript, objects do not have to match a specific, exact type. For instance, if we create an object that fulfills an interface's requirements, we can utilize that object in places where that interface is required, even if there was no explicit connection between them.
Example:

```typescript
type NameProp1 = {
    prop1: string;
};

function log(x: NameProp1) {
    console.log(x.prop1);
}

const obj = {
    prop2: 123,
    prop1: 'Origin',
};

log(obj); // Valid
```

## Namespacing

In TypeScript, namespaces are used to organize code into logical containers, preventing naming collisions and providing a way to group related code together.
The usage of the `export` keywords allows access to the namespace in “outside” modules.

```typescript
export namespace MyNamespace {
    export interface MyInterface1 {
        prop1: boolean;
    }
    export interface MyInterface2 {
        prop2: string;
    }
}

const a: MyNamespace.MyInterface1 = {
    prop1: true,
};
```

## Symbols

Symbols are a primitive data type that represents an immutable value which is guaranteed to be globally unique throughout the lifetime of the program.

Symbols can be used as keys for object properties and provide a way to create non-enumerable properties.

```typescript
const key1: symbol = Symbol('key1');
const key2: symbol = Symbol('key2');

const obj = {
    [key1]: 'value 1',
    [key2]: 'value 2',
};

console.log(obj[key1]); // value 1
console.log(obj[key2]); // value 2
```

## Triple-Slash Directives

Triple-slash directives are special comments that provide instructions to the compiler about how to process a file. These directives begin with three consecutive slashes (///) and are typically placed at the top of a TypeScript file and have no effects on the runtime behavior.

Triple-slash directives are used to reference external dependencies, specify module loading behavior, enable/disable certain compiler features, and more. Few examples:

Referencing a declaration file:

<!-- skip -->
```typescript
/// <reference path="path/to/declaration/file.d.ts" />
```

Indicate the module format:

<!-- skip -->
```typescript
/// <amd|commonjs|system|umd|es6|es2015|none>
```

Enable compiler options, in the following example strict mode:

<!-- skip -->
```typescript
/// <strict|noImplicitAny|noUnusedLocals|noUnusedParameters>
```

## Type Manipulation

### Creating Types from Types

Is it possible to create new types composing, manipulating or transforming existing types.

Intersection Types (`&`):

Allow you to combine multiple types into a single  type:

```typescript
type A = { foo: number };
type B = { bar: string };
type C = A & B; // Intersection of A and B
const obj: C = { foo: 42, bar: 'hello' };
```

Union Types (`|`):

Allow you to define a type that can be one of several types:

```typescript
type Result = string | number;
const value1: Result = 'hello';
const value2: Result = 42;
```

Mapped Types:

Allow you to transform the properties of an existing type to create new type:

```typescript
type Mutable<T> = {
    readonly [P in keyof T]: T[P];
};
type Person = {
    name: string;
    age: number;
};
type ImmutablePerson = Mutable<Person>; // properties become read-only
```

Conditional types:

Allow you to create types based on some conditions:

```typescript
type ExtractParam<T> = T extends (param: infer P) => any ? P : never;
type MyFunction = (name: string) => number;
type ParamType = ExtractParam<MyFunction>; // string
```

### Indexed Access Types

In TypeScript is it possible to access and manipulate  the types of properties within another type using an index, `Type[Key]`.

```typescript
type Person = {
    name: string;
    age: number;
};

type AgeType = Person['age']; // number
```

```typescript
type MyTuple = [string, number, boolean];
type MyType = MyTuple[2]; // boolean
```

### Utility Types

Several built-in utility types can be used to manipulate types, below a list of the most common used:

#### Awaited\<T\>

Constructs a type recursively unwrap Promises.

```typescript
type A = Awaited<Promise<string>>; // string
```

#### Partial\<T\>

Constructs a type with all properties of T set to optional.

```typescript
type Person = {
    name: string;
    age: number;
};

type A = Partial<Person>; // { name?: string | undefined; age?: number | undefined; }
```

#### Required\<T\>

Constructs a type with all properties of T set to required.

```typescript
type Person = {
    name?: string;
    age?: number;
};

type A = Required<Person>; // { name: string; age: number; }
```

#### Readonly\<T\>

Constructs a type with all properties of T set to readonly.

<!-- skip -->
```typescript
type Person = {
    name: string;
    age: number;
};

type A = Readonly<Person>;

const a: A = { name: 'Simon', age: 17 };
a.name = 'John'; // Invalid
```

#### Record\<K, T\>

Constructs a type with a set of properties K of type T.

```typescript
type Product = {
    name: string;
    price: number;
};

const products: Record<string, Product> = {
    apple: { name: 'Apple', price: 0.5 },
    banana: { name: 'Banana', price: 0.25 },
};

console.log(products.apple); // { name: 'Apple', price: 0.5 }
```

#### Pick\<T, K\>

Constructs a type by picking the specified properties K from T.

```typescript
type Product = {
    name: string;
    price: number;
};

type Price = Pick<Product, 'price'>; // { price: number; }
```

#### Omit\<T, K\>

Constructs a type by omitting the specified properties K from T.

```typescript
type Product = {
    name: string;
    price: number;
};

type Name = Omit<Product, 'price'>; // { name: string; }
```

#### Exclude\<T, U\>

Constructs a type by excluding all values of type U from T.

```typescript
type Union = 'a' | 'b' | 'c';
type MyType = Exclude<Union, 'a' | 'c'>; // b
```

#### Extract\<T, U\>

Constructs a type by extracting all values of type U from T.

```typescript
type Union = 'a' | 'b' | 'c';
type MyType = Extract<Union, 'a' | 'c'>; // a | c
```

#### NonNullable\<T\>

Constructs a type by excluding null and undefined from T.

```typescript
type Union = 'a' | null | undefined | 'b';
type MyType = NonNullable<Union>; // 'a' | 'b'
```

#### Parameters\<T\>

Extracts the parameter types of a function type T.

```typescript
type Func = (a: string, b: number) => void;
type MyType = Parameters<Func>; // [a: string, b: number]
```

#### ConstructorParameters\<T\>

Extracts the parameter types of a constructor function type T.

```typescript
class Person {
    constructor(public name: string, public age: number) {}
}
type PersonConstructorParams = ConstructorParameters<typeof Person>; // [name: string, age: number]
const params: PersonConstructorParams = ['John', 30];
const person = new Person(...params);
console.log(person); // Person { name: 'John', age: 30 }
```

#### ReturnType\<T\>

Extracts the return type of a function type T.

```typescript
type Func = (name: string) => number;
type MyType = ReturnType<Func>; // number
```

#### InstanceType\<T\>

Extracts the instance type of a class type T.

```typescript
class Person {
    name: string;

    constructor(name: string) {
        this.name = name;
    }

    sayHello() {
        console.log(`Hello, my name is ${this.name}!`);
    }
}

type PersonInstance = InstanceType<typeof Person>;

const person: PersonInstance = new Person('John');

person.sayHello(); // Hello, my name is John!
```

#### ThisParameterType\<T\>

Extracts the type of 'this' parameter from a function type T.

```typescript
interface Person {
    name: string;
    greet(this: Person): void;
}
type PersonThisType = ThisParameterType<Person['greet']>; // Person
```

#### OmitThisParameter\<T\>

Removes the 'this' parameter from a function type T.

```typescript
function capitalize(this: String) {
    return this[0].toUpperCase + this.substring(1).toLowerCase();
}

type CapitalizeType = OmitThisParameter<typeof capitalize>; // () => string
```

#### ThisType\<T\>

Servers as a market for a contextual `this` type.

<!-- skip -->
```typescript
type Logger = {
    log: (error: string) => void;
};

let helperFunctions: { [name: string]: Function } & ThisType<Logger> = {
    hello: function () {
        this.log('some error'); // Valid as "log" is a part of "this".
        this.update(); // Invalid
    },
};
```

#### Uppercase\<T\>

Make uppercase the name of the input type T.

```typescript
type MyType = Uppercase<'abc'>; // "ABC"
```

#### Lowercase\<T\>

Make lowercase the name of the input type T.

```typescript
type MyType = Lowercase<'ABC'>; // "abc"
```

#### Capitalize\<T\>

Capitalize the name of the input type T.

```typescript
type MyType = Capitalize<'abc'>; // "Abc"
```

#### Uncapitalize\<T\>

Uncapitalize the name of the input type T.

```typescript
type MyType = Uncapitalize<'Abc'>; // "abc"
```

## Others

### Errors and Exception Handling

TypeScript allows you to catch and handle errors using standard JavaScript error handling mechanisms:

Try-Catch-Finally Blocks:

```typescript
try {
    // code that might throw an error
} catch (error) {
    // handle the error
} finally {
    // code that always executes, finally is optional
}
```

You can also handle different types of error:

```typescript
try {
    // code that might throw different types of errors
} catch (error) {
    if (error instanceof TypeError) {
        // handle TypeError
    } else if (error instanceof RangeError) {
        // handle RangeError
    } else {
        // handle other errors
    }
}
```

Custom Error Types:

It is possible to specify more specific error by extending on the Error `class`:

```typescript
class CustomError extends Error {
    constructor(message: string) {
        super(message);
        this.name = 'CustomError';
    }
}

throw new CustomError('This is a custom error.');
```

Mixin classes allow you to combine and compose behavior from multiple classes into a single class. They provide a way to reuse and extend functionality without the need for deep inheritance chains.

```typescript
abstract class Identifiable {
    name: string = '';
    logId() {
        console.log('id:', this.name);
    }
}
abstract class Selectable {
    selected: boolean = false;
    select() {
        this.selected = true;
        console.log('Select');
    }
    deselect() {
        this.selected = false;
        console.log('Deselect');
    }
}
class MyClass {
    constructor() {}
}

// extend MyClass to include the behavior of Identifiable and Selectable
interface MyClass extends Identifiable, Selectable {}

// function to apply mixins to a class
function applyMixins(source: any, baseCtors: any[]) {
    baseCtors.forEach(baseCtor => {
        Object.getOwnPropertyNames(baseCtor.prototype).forEach(name => {
            let descriptor = Object.getOwnPropertyDescriptor(
                baseCtor.prototype,
                name
            );
            if (descriptor) {
                Object.defineProperty(source.prototype, name, descriptor);
            }
        });
    });
}

// apply the mixins to MyClass
applyMixins(MyClass, [Identifiable, Selectable]);
let o = new MyClass();
o.name = 'abc';
o.logId();
o.select();
```

### Asynchronous Language Features

As TypeScript is a superset of JavaScript, it has built-in asynchronous language features of JavaScript as:

Promises:
Promises are a way to handle asynchronous operations and their results using methods like `.then()` and `.catch()` to handle success and error conditions.

To learn more: <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise>

Async/await:
Async/await keywords are a way to provide a more synchronous-looking syntax for working with Promises. The `async` keyword is used to define an asynchronous function, and the `await` keyword is used within an async function to pause execution until a Promise is resolved or rejected.

To learn more:
<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function>
<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await>

The following API are well supported in TypeScript:

Fetch API
<https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API>

Web Workers
<https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API>

Shared Workers
<https://developer.mozilla.org/en-US/docs/Web/API/SharedWorker>

WebSocket
<https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API>

### Iterators and Generators

Both Interators and Generators are well supported in TypeScript.

Iterators are objects that implement the iterator protocol, providing a way to access elements of a collection or sequence one by one. It is a structure that contains a pointer to the next element in the iteration. They have a `next()` method that returns the next value in the sequence along with a boolean indicating if the sequence is `done`.

```typescript
class NumberIterator implements Iterable<number> {
    private current: number;

    constructor(private start: number, private end: number) {
        this.current = start;
    }

    public next(): IteratorResult<number> {
        if (this.current <= this.end) {
            const value = this.current;
            this.current++;
            return { value, done: false };
        } else {
            return { value: undefined, done: true };
        }
    }

    [Symbol.iterator](): Iterator<number> {
        return this;
    }
}

const iterator = new NumberIterator(1, 3);

for (const num of iterator) {
    console.log(num);
}
```

Generators are special functions defined using the `function*` syntax that simplifies the creation of iterators. They use the `yield` keyword to define the sequence of values and automatically pause and resume execution when values are requested.

Generators make it easier to create iterators and are especially useful for working with large or infinite sequences.

Example:

```typescript
function* numberGenerator(start: number, end: number): Generator<number> {
    for (let i = start; i <= end; i++) {
        yield i;
    }
}

const generator = numberGenerator(1, 5);

for (const num of generator) {
    console.log(num);
}
```

TypeScript also supports async iterators and async Generators.

To learn more:
<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator>        <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Iterator>

### TsDocs JSDoc Reference

When working with a JavaScript code base, it is possible to help TypeScript to infer the right Type by using JSDoc comments with additional annotation to provide type information.

Example:

```typescript
/**
 * Computes the power of a given number
 * @constructor
 * @param {number} base – The base value of the expression
 * @param {number} exponent – The exponent value of the expression
 */
function power(base: number, exponent: number) {
    return Math.pow(base, exponent);
}
power(10, 2); // function power(base: number, exponent: number): number
```

Full documentation is provided to this link:
<https://www.typescriptlang.org/docs/handbook/jsdoc-supported-types.html>

From version 3.7 it is possible to generate .d.ts type definitions from JavaScript JSDoc syntax.
More information can be found here:
<https://www.typescriptlang.org/docs/handbook/declaration-files/dts-from-js.html>

### @types

Packages under the @types organization are special package naming conventions used to provide type definitions for existing JavaScript libraries or modules. For instance using:

```shell
npm install --save-dev @types/lodash
```

Will install the type definitions of `lodash` in your current project.

To contribute to the type definitions of @types package, please submit a pull request to <https://github.com/DefinitelyTyped/DefinitelyTyped>.

### JSX

JSX (JavaScript XML) is an extension to the JavaScript language syntax that allows you to write HTML-like code within your JavaScript or TypeScript files. It is commonly used in React to define the HTML structure.

TypeScript extends the capabilities of JSX by providing type checking and static analysis.

To use JSX you need to set the `jsx` compiler option in your `tsconfig.json` file. Two common configuration options:

* "preserve": emit .jsx files with the JSX unchanged. This option tells TypeScript to keep the JSX syntax as-is and not transform it during the compilation process. You can use this option if you have a separate tool, like Babel, that handles the transformation.
* "react": enables TypeScript's built-in JSX transformation. React.createElement will be used.

All options are available here:
<https://www.typescriptlang.org/tsconfig#jsx>

### ES6 Modules

TypeScript does support ES6 (ECMAScript 2015) and many subsequent versions. This means you can use ES6 syntax, such as arrow functions, template literals, classes, modules, destructuring, and more.

To enable ES6 features in your project, you can specify the `target` property in the tsconfig.json.

A configuration example:

```json
{
  "compilerOptions": {
    "target": "es6",
    "module": "es6",
    "moduleResolution": "node",
    "sourceMap": true,
    "outDir": "dist"
  },
  "include": ["src"]
}
```

### ES7 exponentiation operator

The exponentiation (**) operator computes the value obtained by raising the first operand to the power of the second operand. It functions similarly to Math.pow(), but with the added capability of accepting BigInts as operands.
TypeScript fully supports this operator using as `target` in your tsconfig.json file `es2016` or larger version.

```typescript
console.log(2 ** (2 ** 2)); // 16
```

### The for-await-of Statement

This is a JavaScript feature fully supported in TypeScript which allows you to iterate over asynchronous iterable objects from target version es2018.

```typescript
async function* asyncNumbers(): AsyncIterableIterator<number> {
    yield Promise.resolve(1);
    yield Promise.resolve(2);
    yield Promise.resolve(3);
}

(async () => {
    for await (const num of asyncNumbers()) {
        console.log(num);
    }
})();
```

### New.target

You can use in TypeScript the `new.target` meta-property which enables you to determine if a function or constructor was invoked using the new operator. It allows you to detect whether an object was created as a result of a constructor call.

```typescript
class Parent {
    constructor() {
        debugger;
        console.log(new.target); // logs the constructor function used to create an instance
    }
}

class Child extends Parent {
    constructor() {
        super();
    }
}

const parentX = new Parent(); // [Function: Parent]
const child = new Child(); // [Function: Child]
```

### Dynamic Import Expressions

It is possible to conditionally load modules or lazy load them on-demand using the ECMAScript proposal for dynamic import which is supported in TypeScript.

The syntax for dynamic import expressions in TypeScript is as follows:

<!-- skip -->
```typescript
async function renderWidget() {
    const container = document.getElementById('widget');
    if (container !== null) {
        const widget = await import('./widget'); // dynamic import
        widget.render(container);
    }
}

renderWidget();
```

### “tsc –watch”

This command starts a TypeScript compiler with --watch parameter, with the ability to automatically recompile TypeScript files whenever they are modified.

```shell
tsc --watch
```

Starting from TypeScript version 4.9, file monitoring primarily relies on file system events, automatically resorting to polling if an event-based watcher cannot be established.

### Definite Assignment Assertions (!)

The Definite Assignment Assertions or also called non-null assertion operator tells the TypeScript compiler that a value typed cannot be null or undefined which is a way to override the compiler's analysis and inform it that a variable will be assigned a value before it is used.

```typescript
type Person = {
    name: string;
};

const printName = (person?: Person) => {
    console.log(`Name is ${person!.name}`);
};
```

### Defaulted declarations

Defaulted declarations are used when a variable or parameter is assigned a default value. This means that if no value is provided for that variable or parameter, the default value will be used instead.

```typescript
function greet(name: string = 'Anonymous'): void {
    console.log(`Hello, ${name}!`);
}
greet(); // Hello, Anonymous!
greet('John'); // Hello, John!
```

### “const“ assertions

Const assertions are a feature that allows you to declare a variable with a more specific literal type based on its initialization value. It is a way to state to the compiler that the value has to be treated as an immutable literal.

<!-- skip -->
```typescript
let arr = [1, 2, 3] as const; // readonly [1, 2, 3]
arr.push(4); // Invalid
```

### Optional Chaining

The optional chaining operator ?. works like the regular dot operator (.) for accessing properties or methods. However, it gracefully handles null or undefined values by terminating the expression and returning undefined, instead of throwing an error.

```typescript
type Person = {
    name: string;
    age?: number;
    address?: {
        street?: string;
        city?: string;
    };
};

const person: Person = {
    name: 'John',
};

console.log(person.address?.city); // undefined
```

### Nullish coalescing operator (??)

The nullish coalescing operator `??` returns the right-hand side value if the left-hand side is `null` or `undefined`; otherwise, it returns the left-hand side value.

```typescript
const foo = null ?? 'foo';
console.log(foo); // foo

const baz = 1 ?? 'baz';
const baz2 = 0 ?? 'baz';
console.log(baz); // 1
console.log(baz2); // 0
```

### Template Literal Types

Template Literal Types allow to manipulate string value at type level and generate new string types based on existing ones. They are useful to create more expressive and precise types from string-based operations.

```typescript
type Department = 'engineering' | 'hr';
type Language = 'english' | 'spanish';
type Id = `${Department}-${Language}-id`; // "engineering-english-id" | "engineering-spanish-id" | "hr-english-id" | "hr-spanish-id"
```

### Function overloading

Function overloading allows you to define multiple function signatures for the same function name, each with different parameter types and return type.
When you call an overloaded function, TypeScript uses the provided arguments to determine the correct function signature:

```typescript
function makeGreeting(name: string): string;
function makeGreeting(names: string[]): string[];

function makeGreeting(person: unknown): unknown {
    if (typeof person === 'string') {
        return `Hi ${person}!`;
    } else if (Array.isArray(person)) {
        return person.map(name => `Hi, ${name}!`);
    }
    throw new Error('Unable to greet');
}

makeGreeting('Simon');
makeGreeting(['Simone', 'John']);
```

### Recursive Conditional Types

It is possible to define complex type relationships using logic and recursion in TypeScript.
Let’s break it down in simple terms:

Conditional Types: allows you to define types based on boolean conditions:

```typescript
type CheckNumber<T> = T extends number ? 'Number' : 'Not a number';
type A = CheckNumber<123>; // 'Number'
type B = CheckNumber<'abc'>; // 'Not a number'
```

Recursion: means a type definition that refers to itself within its own definition:

```typescript
type Json = string | number | boolean | null | Json[] | { [key: string]: Json };

const data: Json = {
    prop1: true,
    prop2: 'prop2',
    prop3: {
        prop4: [],
    },
};
```

Recursive Conditional Types combine both conditional logic and recursion. It means that a type definition can depend on itself through conditional logic, creating complex and flexible type relationships.

```typescript
type Flatten<T> = T extends Array<infer U> ? Flatten<U> : T;

type NestedArray = [1, [2, [3, 4], 5], 6];
type FlattenedArray = Flatten<NestedArray>; // 2 | 3 | 4 | 5 | 1 | 6
```

### ECMAScript Module Support in Node.js

Node.js added support for ECMAScript Modules starting from version 15.3.0, and TypeScript has had ECMAScript Module Support for Node.js since version 4.7. This support can be enabled by using the `module` property with the value `nodenext` in the tsconfig.json file. Here's an example:

```json
{
  "compilerOptions": {
    "module": "nodenext",
    "outDir": "./lib",
    "declaration": true
  }
}
```

Node.js supports two file extensions for modules: `.mjs` for ES modules and `.cjs` for CommonJS modules. The equivalent file extensions in TypeScript are `.mts` for ES modules and `.cts` for CommonJS modules. When the TypeScript compiler transpiles these files to JavaScript, it will create `.mjs` and `.cjs` files.

If you want to use ES modules in your project, you can set the `type` property to "module" in your package.json file. This instructs Node.js to treat the project as an ES module project.

Additionally, TypeScript also supports type declarations in .d.ts files. These declaration files provide type information for libraries or modules written in TypeScript, allowing other developers to utilize them with TypeScript's type checking and auto-completion features.

### Assertion Functions

In TypeScript, assertion functions are functions that indicate the verification of a specific condition based on their return value. In their simplest form, an assert function examines a provided predicate and raises an error when the predicate evaluates to false.

```typescript
function isNumber(value: unknown): asserts value is number {
    if (typeof value !== 'number') {
        throw new Error('Not a number');
    }
}
```

Or can be declared as function expression:

```typescript
type AssertIsNumber = (value: unknown) => asserts value is number;
const isNumber: AssertIsNumber = value => {
    if (typeof value !== 'number') {
        throw new Error('Not a number');
    }
};
```

Assertion functions share similarities with type guards. Type guards were initially introduced to perform runtime checks and ensure the type of a value within a specific scope.
Specifically, a type guard is a function that evaluates a type predicate and returns a boolean value indicating whether the predicate is true or false. This differs slightly from assertion functions,where the intention is to throw an error rather than returning false when the predicate is not satisfied.

Example of type guard:

```typescript
const isNumber = (value: unknown): value is number => typeof value === 'number';
```

### Variadic Tuple Types

Variadic Tuple Types are a features introduces in TypeScript version 4.0, let’s start to learn them by revise what is a tuple:

A tuple type is an array which has a defined length, and were the type of each element is known:

```typescript
type Student = [string, number];
const [name, age]: Student = ['Simone', 20];
```

The term “variadic” means indefinite arity (accept a variable number of arguments).

A variadic tuple is a tuple type which has all the property as before but the exact shape is not defined yet:

```typescript
type Bar<T extends unknown[]> = [boolean, ...T, number];

type A = Bar<[boolean]>; // [boolean, boolean, number]
type B = Bar<['a', 'b']>; // [boolean, 'a', 'b', number]
type C = Bar<[]>; // [boolean, number]
```

In the previous code we can see that the tuple shape is defined by the `T` generic passed in.

Variadic tuples can accept multiple generics make them very flexible:

```typescript
type Bar<T extends unknown[], G extends unknown[]> = [...T, boolean, ...G];

type A = Bar<[number], [string]>; // [number, boolean, string]
type B = Bar<['a', 'b'], [boolean]>; // ["a", "b", boolean, boolean]
```

With the new variadic tuples we can use:

* The spreads in tuple type syntax can now be generic, so we can represent higher-order operation on tuples and arrays even when we do not know the actual types we are operating over.
* The rest elements can occur anywhere in a tuple.

Example:

```typescript
type Items = readonly unknown[];

function concat<T extends Items, U extends Items>(
    arr1: T,
    arr2: U
): [...T, ...U] {
    return [...arr1, ...arr2];
}

concat([1, 2, 3], ['4', '5', '6']); // [1, 2, 3, "4", "5", "6"]
```

### Boxed types

Boxed types refer to the wrapper objects that are used to represent primitive types as objects. These wrapper objects provide additional functionality and methods that are not available directly on the primitive values.

When you access a method like `charAt` or `normalize` on a `string` primitive, JavaScript wraps it in a `String` object, calls the method, and then throws the object away.

Demonstration:

```typescript
const originalNormalize = String.prototype.normalize;
String.prototype.normalize = function () {
    console.log(this, typeof this);
    return originalNormalize.call(this);
};
console.log('\u0041'.normalize());
```

TypeScript represents this differentiation by providing separate types for the primitives and their corresponding object wrappers:

string => String
number => Number
boolean => Boolean
symbol => Symbol
bigint => BigInt

The boxed types are usually not needed. Avoid using boxed types and instead use type for the primitives,  for instance `string` instead of `String`.

### Key Remapping in Mapped Types

Mapped types allow you to create new types by transforming the properties of an existing type. Using the `keyof` and `in` keywords, you can iterate over the properties of a type and define modifications, such as making them optional or readonly. Here an example:

```typescript
type Person = {
    name: string;
    age: number;
    email: string;
};

type PartialPerson = {
    [K in keyof Person]?: Person[K]; // This will make all properties are optional
};

const partialPerson: PartialPerson = {
    name: 'John',
    age: 30,
};

partialPerson.email = 'john@example.com';
```

### Covariance and Contravariance in TypeScript

Covariance and Contravariance are used to describe how relationships work when dealing with inheritance or assignment of types.

Covariance means that a type relationship preserves the direction of inheritance or assignment, so if a type A is a subtype of type B, then an array of type A is also considered a subtype of an array of type B. The important thing to note here is that the subtype relationship is maintained this means that Covariance accept subtype but doesn't accept supertype.

Contravariance means that a type relationship reverses the direction of inheritance or assignment, so if a type A is a subtype of type B, then an array of type B is considered a subtype of an array of type A. The subtype relationship is reversed this means that Contravariance accept supertype but doesn't accept subtype.

Notes: Bivariance means accept both supertype & subtype.

Example: Let's say we have a space for all animals and a separate space just for dogs.

In Covariance, you can put all the dogs in the animals space because dogs are a type of animal. But you cannot put all the animals in the dog space because there might be other animals mixed in.

In Contravariance, you cannot put all the animals in the dogs space because the animals space might contain other animals as well. However, you can put all the dogs in the animal space because all dogs are also animals.

<!-- skip -->
```typescript
// Covariance example
class Animal {
    name: string;
    constructor(name: string) {
        this.name = name;
    }
}

class Dog extends Animal {
    breed: string;
    constructor(name: string, breed: string) {
        super(name);
        this.breed = breed;
    }
}

let animals: Animal[] = [];
let dogs: Dog[] = [];

// Covariance allows assigning subtype (Dog) array to supertype (Animal) array
animals = dogs;
dogs = animals; // Invalid: Type 'Animal[]' is not assignable to type 'Dog[]'

// Contravariance example
type Feed<in T> = (animal: T) => void;

let feedAnimal: Feed<Animal> = (animal: Animal) => {
    console.log(`Animal name: ${animal.name}`);
};

let feedDog: Feed<Dog> = (dog: Dog) => {
    console.log(`Dog name: ${dog.name}, Breed: ${dog.breed}`);
};

// Contravariance allows assigning supertype (Animal) callback to subtype (Dog) callback
feedDog = feedAnimal;
feedAnimal = feedDog; // Invalid: Type 'Feed<Dog>' is not assignable to type 'Feed<Animal>'.
```

In TypeScript, type relationships for arrays are covariant, while type relationships for function parameters are contravariant. This means that TypeScript exhibits both covariance and contravariance, depending on the context.

#### Optional Variance Annotations for Type Parameters

As of TypeScript 4.7.0, we can use the `out` and `in` keywords to be specific about Variance annotation.

For Covariant, use the `out` keyword:

```typescript
type AnimalCallback<out T> = () => T; // T is Covariant here
```

And for Contravariant, use the `in` keyword:

```typescript
type AnimalCallback<in T> = (value: T) => void; // T is Contravariance here
```

### Symbol and Template String Pattern Index Signatures

Symbols are unique identifiers that can be used as property keys in objects to prevent naming conflicts.

Template string pattern index signatures allow us to define flexible index signatures using template string patterns. This feature enables us to create objects that can be indexed with specific patterns of string keys, providing more control and specificity when accessing and manipulating properties.

TypeScript from version 4.4 allows index signatures for symbols and template string patterns.

```typescript
type Obj = {
    [sym: symbol]: number;
};

const a = Symbol('a');
const b = Symbol('b');

let obj: Obj = {};

obj[b] = 123;
```

### The satisfies Operator

The `satisfies`  allows you to check if a given type satisfies a specific interface or condition. In other words, it ensures that a type has all the required properties and methods of a specific interface. It is a way to ensure a variable fits into a definition of a type
Here is an example:

<!-- skip -->
```typescript
type Columns = 'name' | 'nickName' | 'attributes';

type User = Record<Columns, string | string[] | undefined>;

// Type Annotation using `User`
const user: User = {
    name: 'Simone',
    nickName: undefined,
    attributes: ['dev', 'admin'],
};

// In the following lines, TypeScript won't be able to infer properly
user.attributes?.map(console.log); // Property 'map' does not exist on type 'string | string[]'. Property 'map' does not exist on type 'string'.
user.nickName; // string | string[] | undefined

// Type assertion using `as`
const user2 = {
    name: 'Simon',
    nickName: undefined,
    attributes: ['dev', 'admin'],
} as User;

// Here too, TypeScript won't be able to infer properly
user2.attributes?.map(console.log); // Property 'map' does not exist on type 'string | string[]'. Property 'map' does not exist on type 'string'.
user2.nickName; // string | string[] | undefined

// Using `satisfies` operators we can properly infer the types now
const user3 = {
    name: 'Simon',
    nickName: undefined,
    attributes: ['dev', 'admin'],
} satisfies User;

user3.attributes?.map(console.log); // TypeScript infers correctly: string[]
user3.nickName; // TypeScript infers correctly: undefined
```