07_Functions.md

Function Types

General syntax

function fun(arg1: type, arg2 = 'str', arg3?: string): returntype {
  // (mostly typescript guesses the correct return type using type inference)
    return matches returntype
}

Optional parameters are dentoted with (?). Optional parameters must be after required parameters.

Default parameters are assigned a default value (and their types are inferred from default value). It is not necessary to specify the type of default parameters.

Function Declaration

Just defining the function type without implementation.

function add(a: string, b: string): string;

A function that returns nothing has a return type of void.

function sum(num1: number, num2: number): void {
  // return num1 + num2; // Error: Type 'number' is not assignable to type 'void'.
}

Function Expression or Anonymous Function

Parameter names are just for reference not important to match paramter names.

let sum2: (num1: number, num2: number) => number;

sum2 = (number1, number2) => number1 + number2;

Custom Function Type

type Sum = (num1: number, num2: number) => number;

// Function Expression
let sum3: Sum = (number1, number2) => number1 + number2;

let sum4: typeof sum;

Generic Function Type

Generic type can be used to create a function that can work with a variety of types.

let fun: Function;

fun = (a: number, b: number) => a + b;

fun = (a: string, b: string) => a + b;

Function Overloads

Function overloads allow you to work with multiple parameters and types. As typescript can't distinguish the return type of function with multiple types

type Combinable = number | string;

function add(a: string): string;
function add(a: string, b: string): string;
function add(a: number, b: string): string;
function add(a: string, b: number): string;
function add(a: number, b: number): number;
function add(a: Combinable, b?: Combinable) {
  if (!b) {
    return a.toString();
  }

  if (typeof a === "number" && typeof b === "number") return a + b;

  return a.toString() + b.toString();
}

const result = add("john", 5); // without overloading the return type will be string | number

result.split("");