TypeScript Introduction

TypeScript is a strongly typed superset of JavaScript that compiles to plain JavaScript.

Why TypeScript?

• Type Safety: Catch errors at compile time, not runtime
• Better IDE support: Autocompletion, refactoring, navigation
• Self-documenting: Types serve as inline documentation
• Scalability: Essential for large codebases
• Modern JS: All ES6+ features + types

Basic Types

// Primitives
let name: string = "Alice";
let age: number = 30;
let active: boolean = true;
let nothing: null = null;
let notDefined: undefined = undefined;

// Arrays
let scores: number[] = [95, 87, 92];
let langs: Array<string> = ["TypeScript", "Python"];

// Tuple
let pair: [string, number] = ["Alice", 30];

// Any (avoid!) and Unknown (safer)
let data: unknown = fetchData();

// Union types
let id: string | number = "abc123";

// Literal types
let direction: "north" | "south" | "east" | "west" = "north";
let status: 200 | 201 | 400 | 404 | 500 = 200;

Interfaces

interface User {
  id: string;
  name: string;
  email: string;
  age?: number;        // optional
  readonly createdAt: Date; // readonly
}

interface Admin extends User {
  role: "admin";
  permissions: string[];
}

// Usage
const user: User = {
  id: "u_001",
  name: "Alice",
  email: "alice@example.com",
  createdAt: new Date(),
};

Type Aliases

// Object type
type Point = { x: number; y: number };

// Function type
type Transformer<T> = (value: T) => T;

// Union
type StringOrNumber = string | number;

// Intersection
type AdminUser = User & { role: "admin" };

Generics

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

identity<string>("hello"); // type: string
identity<number>(42);      // type: number

// Generic interface
interface ApiResponse<T> {
  data: T;
  status: number;
  message: string;
}

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

Utility Types

interface User {
  id: string;
  name: string;
  email: string;
  password: string;
}

// Partial - all optional
type UpdateUser = Partial<User>;

// Required - all required
type FullUser = Required<User>;

// Pick - select fields
type PublicUser = Pick<User, "id" | "name" | "email">;

// Omit - exclude fields
type SafeUser = Omit<User, "password">;

// Readonly
type ImmutableUser = Readonly<User>;

// Record
type RoleMap = Record<string, string[]>;