Dart Programming Cheatsheet

Comprehensive Dart cheatsheet with TOC covering syntax, variables, operators, control flow, functions, collections, OOP, null safety, async/await, streams, exceptions, libraries, and Dart 3 features (records, patterns). Perfect quick reference for beginners and pros.

Table of Contents

1. Basics
2. Variables & Data Types
3. Operators
4. Control Flow
5. Functions
6. Collections
7. Object-Oriented Programming (OOP)
8. Exception Handling
9. Asynchronous Programming
10. Libraries & Imports
11. Null Safety
12. Common Useful Features

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1. Basics

Hello World

void main() {
  print('Hello, Dart!');
}

Entry Point

• main() is the required entry point.
• Use void main(List<String> args) to accept command-line arguments.

Comments

// Single-line comment

/*
  Multi-line comment
*/

/// Documentation comment (for dartdoc)

---

2. Variables & Data Types

Variable Declaration

var name = 'Alice';          // Type inferred
String name = 'Alice';       // Explicit type
dynamic anything = 42;       // Can change type
Object something = 'hi';     // Base class

Late Initialization

late String description;   // Initialized later
description = 'now set';

Constants

const pi = 3.14;           // Compile-time constant
final answer = 42;         // Run-time constant (set once)

Built-in Types

Type	Example
int	int age = 30;
double	double price = 9.99;
num (int/double)	num x = 10; x = 10.5;
String	String name = 'Dart';
bool	bool isReady = true;
List	List<int> nums = [1, 2, 3];
Set	Set<String> names = {'a', 'b'};
Map	Map<String, int> ages = {'Bob': 25};
Runes	Unicode code points
Symbols	#mySymbol (rare)

String Interpolation

int age = 25;
print('I am $age years old');     // I am 25 years old
print('Next year: ${age + 1}');    // Next year: 26

---

3. Operators

Arithmetic

+   -   *   /       // Basic
~/          // Integer division
%           // Modulus
++   --     // Increment/decrement

Comparison & Logical

==   !=   >   <   >=   <=
&&   ||   !             // AND, OR, NOT

Type Test

is    // True if object has type
is!   // True if object does NOT have type
as    // Type cast

Assignment & Null-aware

??=       // Assign if null
??        // If null, use alternative
?.        // Safe navigation
...       // Spread operator

Example:

int? a;
int b = a ?? 10;       // b = 10
a ??= 5;               // a = 5
print(a?.isEven);      // Safe access

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4. Control Flow

If-Else

if (condition) {
  // ...
} else if (other) {
  // ...
} else {
  // ...
}

Switch

switch (value) {
  case 1:
    print('one');
    break;
  case 2:
    print('two');
    break;
  default:
    print('other');
}
// Dart 3.0: Switch expressions
var result = switch (value) { 1 => 'one', 2 => 'two', _ => 'other' };

Loops

// For loop
for (int i = 0; i < 5; i++) { print(i); }

// For-in
var list = [1, 2, 3];
for (var item in list) { print(item); }

// ForEach (functional)
list.forEach((item) => print(item));

// While / Do-while
while (condition) { }
do { } while (condition);

Break & Continue

break;    // Exit loop/switch
continue; // Skip to next iteration

---

5. Functions

Basic Function

int add(int a, int b) {
  return a + b;
}

One-line (fat arrow)

int add(int a, int b) => a + b;

Optional Parameters

// Positional optional (must be in order, default null or value)
void greet(String name, [String? title]) {
  print('$title $name');
}
greet('Alice', 'Ms.');

// Named parameters (with defaults or required)
void register({required String name, int age = 0}) {
  print('$name, $age');
}
register(name: 'Bob', age: 25);

Anonymous Function (Lambda)

var square = (int x) => x * x;
print(square(5)); // 25

Functions as First-class Objects

void doSomething(void Function() callback) {
  callback();
}

Lexical Closures

Function makeCounter() {
  int count = 0;
  return () => count++;
}

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6. Collections

List (Array)

var list = [1, 2, 3];
list.add(4);               // [1,2,3,4]
list.removeAt(0);          // [2,3,4]
list[1] = 99;              // [2,99,4]
print(list.length);        // 3

// List literals with spread/collection if
var other = [0, ...list];
var withIf = [1, if (condition) 2];
var withFor = [for (var i in list) i * 2];

Set (Unique items)

var set = {1, 2, 3};
set.add(2);          // No duplicate
set.add(4);
print(set.contains(3)); // true

Map (Key-Value)

var map = {'a': 1, 'b': 2};
map['c'] = 3;
print(map['a']);     // 1
map.forEach((k, v) => print('$k: $v'));

Collection Methods (Where, Map, Reduce)

var nums = [1, 2, 3, 4];
var evens = nums.where((n) => n % 2 == 0);   // (2,4)
var squares = nums.map((n) => n * n);        // (1,4,9,16)
var sum = nums.reduce((a, b) => a + b);      // 10
var hasBig = nums.any((n) => n > 3);          // true

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7. Object-Oriented Programming (OOP)

Class Definition

class Person {
  String name;
  int age;

  // Constructor
  Person(this.name, this.age);
  
  // Named constructor
  Person.guest() : name = 'Guest', age = 0;
  
  // Method
  void greet() => print('Hi, I am $name');
  
  // Getter/Setter
  String get display => '$name ($age)';
  set setAge(int a) => age = a;
}

Inheritance

class Student extends Person {
  int grade;
  Student(String name, int age, this.grade) : super(name, age);
  
  @override
  void greet() => print('I am a student');
}

Abstract Class & Interface

abstract class Animal {
  void makeSound();  // Abstract method
}

class Dog implements Animal {
  @override
  void makeSound() => print('Woof');
}

Mixins

mixin Flyable {
  void fly() => print('Flying');
}

class Bird with Flyable {}

Static Members

class MathUtil {
  static const pi = 3.14;
  static int add(int a, int b) => a + b;
}
print(MathUtil.pi);

---

8. Exception Handling

Try-Catch-Finally

try {
  int result = 100 ~/ 0;
} catch (e, stackTrace) {
  print('Error: $e');
  print(stackTrace);
} finally {
  print('Always runs');
}

Specific Catch

try {
  // ...
} on IntegerDivisionByZeroException {
  print('Cannot divide by zero');
} catch (e) {
  print('Other error: $e');
}

Throw

void validateAge(int age) {
  if (age < 0) throw Exception('Age cannot be negative');
}

---

9. Asynchronous Programming

Future & async/await

Future<String> fetchData() async {
  await Future.delayed(Duration(seconds: 2));
  return 'Data loaded';
}

void main() async {
  print('Loading...');
  String data = await fetchData();
  print(data);
}

Handle Errors with async/await

try {
  var result = await riskyOperation();
} catch (e) {
  print('Failed: $e');
}

Future API (then, catchError)

fetchData().then((value) {
  print(value);
}).catchError((error) {
  print(error);
});

Streams

Stream<int> countStream(int max) async* {
  for (int i = 1; i <= max; i++) {
    await Future.delayed(Duration(seconds: 1));
    yield i;
  }
}

void main() async {
  await for (var value in countStream(3)) {
    print(value);
  }
}

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10. Libraries & Imports

Import

import 'dart:io';          // Core library
import 'package:http/http.dart' as http;  // Package with alias
import 'my_file.dart';      // Local file

Export parts of library

// In library.dart
export 'src/helper.dart' show usefulFunction;

Part / Part of (splitting large libraries)

// In main_lib.dart
part 'helper.dart';

// In helper.dart
part of 'main_lib.dart';

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11. Null Safety

Introduction (Dart 2.12+)

• Types are non-nullable by default.
• Add ? to make nullable.

Nullable Types

String? nullableString = null;
int? nullableInt;

Null Assertion Operator (!)

String? maybeName = getUserName();
int length = maybeName!.length; // Assumes not null — dangerous if null

Late Variables

late String description;  // No initial value, but non-nullable
description = 'ready';    // Must set before use

Required Named Parameters

void myFunction({required int value}) { }

Type Promotion

void printLength(String? str) {
  if (str != null) {
    print(str.length);   // str promoted to non-nullable String
  }
}

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12. Common Useful Features

Typedef (type alias)

typedef IntCallback = int Function(int a, int b);
int add(int a, int b) => a + b;
IntCallback calc = add;

Enums (enhanced in Dart 3)

enum Status { pending, success, failed }

// With values/methods
enum Color {
  red('#FF0000'),
  green('#00FF00');
  final String hexCode;
  const Color(this.hexCode);
}

Extension Methods

extension NumberParsing on String {
  int toInt() => int.parse(this);
}
print('123'.toInt());  // 123

Records (Dart 3+)

var record = (name: 'Alice', age: 25);
print(record.name);  // Alice
print(record.$1);    // Alice (positional access)

Patterns (Dart 3+)

var (a, b) = (1, 2);
switch (obj) {
  case [var first, var second]: print('$first, $second');
  case int x when x > 0: print('Positive $x');
}

Cascade Notation (..)

var list = [1, 2, 3]
  ..add(4)
  ..removeAt(0);

Assert

assert(condition, 'Optional error message');
// Only checks in development mode

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Quick Reference Card

Task	Code
Print	print('text');
Null-safe access	obj?.property
List literal	[1, 2, 3]
Map literal	{'a': 1}
Async loop	await for (var x in stream)
Future delay	await Future.delayed(Duration(seconds: 1));
Spread	[...list]
Collection if/for	[if(cond) 1, for(x in list) x]
Class constructor	ClassName(this.prop1, this.prop2);

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