TypeScript Cheat Sheet: Essential Tips for JavaScript Developers

1. Basic Types

let isDone: boolean = false;
let count: number = 42;
let name: string = 'Bryan Lomerio';

1. Arrays

let numbers: number[] = [1, 2, 3];
let names: Array<string> = ['Alice', 'Bob', 'Charlie'];

1. Tuples

let user: [string, number] = ['Bryan', 25];

1. *Enums *

enum Role {
  Admin = 'admin',
  User = 'user',
  Guest = 'guest',
}

let currentRole: Role = Role.Admin;

1. Any Type

let uncertain: any = 42;
uncertain = 'Now a string';
uncertain = true;

1. Void (Function with No Return)

function logMessage(message: string): void {
  console.log(message);
}

1. Null and Undefined

let n: null = null;
let u: undefined = undefined;

1. Type Aliases

type ID = string | number;
let userId: ID = 'abc123';
userId = 42;

1. Interfaces

interface Person {
  name: string;
  age: number;
}

let user: Person = { name: 'Alice', age: 30 };

10. **Functions**

function add(x: number, y: number): number {
  return x + y;
}

const subtract = (x: number, y: number): number => x - y;

1. Classes

class Person {
  name: string;
  age: number;

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

  greet(): void {
    console.log(`Hello, my name is ${this.name}.`);
  }
}

let person = new Person('Bryan', 25);
person.greet();

1. Generics

function identity<T>(value: T): T {
  return value;
}

let stringIdentity = identity<string>('Hello');
let numberIdentity = identity<number>(42);

13. **Type Assertions**

let someValue: any = 'Hello, TypeScript!';
let strLength: number = (someValue as string).length;

1. Union Types

function printId(id: string | number): void {
  console.log(id);
}

printId('abc123');
printId(100);

1. Type Narrowing

function printInfo(value: string | number) {
  if (typeof value === 'string') {
    console.log(`String: ${value}`);
  } else {
    console.log(`Number: ${value}`);
  }
}