Introduction to TypeScript

TypeScript is a superset of JavaScript that adds static typing. You write JavaScript with type annotations, and the TypeScript compiler checks your code for type errors before it runs — then compiles to plain JavaScript that runs anywhere.

// JavaScript — bug only found at runtime
function greet(name) {
  return "Hello, " + name.toUpperCase();
}
greet(42); // 💥 runtime crash: name.toUpperCase is not a function

// TypeScript — error caught at compile time
function greet(name: string) {
  return "Hello, " + name.toUpperCase();
}
greet(42); // ❌ Argument of type 'number' is not assignable to 'string'

Why use it?

• Catch bugs early — type errors, typos, and null issues surface before running.
• Better tooling — autocomplete, inline documentation, safe refactoring.
• Self-documenting — types describe what functions expect and return.
• Scales — invaluable in large codebases and teams.

Basic type annotations

let title: string = "Hello";
let count: number = 42;
let isActive: boolean = true;
let tags: string[] = ["a", "b"];
let pair: [string, number] = ["age", 30]; // tuple

// Functions
function add(a: number, b: number): number {
  return a + b;
}

// Often you can let TypeScript infer the type
let inferred = "TypeScript knows this is a string";