TypeScript Cheatsheet — Types, Interfaces, Generics & More

Basic Types

let name: string = "Alice";
let age: number = 30;
let active: boolean = true;
let items: string[] = ["a", "b"];
let tuple: [string, number] = ["age", 30];
let anything: any = 42;
let unknown: unknown = "?";  // Safer than any
type ID = string | number;   // Union type

Interfaces vs Types

// Interface — for objects, can be extended
interface User {
  id: number;
  name: string;
  email?: string;           // Optional
  readonly createdAt: Date;  // Immutable
}

interface Admin extends User {
  role: 'admin';
}

// Type alias — for unions, intersections, primitives
type Status = 'active' | 'inactive';
type UserWithRole = User & { role: string };
type StringOrNumber = string | number;

Generics

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

interface ApiResponse<T> {
  data: T;
  error?: string;
}

// Constrained generic
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

Utility Types

Type Guards

function isString(val: unknown): val is string {
  return typeof val === 'string';
}

// Discriminated union
type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'rect'; width: number; height: number };

function area(s: Shape) {
  switch (s.kind) {
    case 'circle': return Math.PI * s.radius ** 2;
    case 'rect': return s.width * s.height;
  }
}