TypeScript for JavaScript Developers

Tutorial · Programming Languages

TypeScript is JavaScript with an optional type system layered on top — every valid JS file is already close to valid TypeScript. The value comes from catching a category of bugs at compile time instead of at runtime, in production.

Basic types

let age: number = 30
let name: string = 'Amara'
let isActive: boolean = true
let tags: string[] = ['dev', 'admin']

Most of the time, though, you don't even write these — TypeScript infers them from the initial value, so let age = 30 is already typed as number without the annotation.

Function signatures

function greet(name: string): string {
  return `Hello, ${name}`
}

If you call greet(42), TypeScript flags it before you ever run the code — a whole class of "undefined is not a function" bugs simply can't happen for typed parameters.

Interfaces: shaping objects

interface User {
  id: number
  name: string
  email?: string
}

function printUser(user: User) {
  console.log(user.name)
}

The ? marks email as optional. Any object passed to printUser must have id and name of the right types, or TypeScript won't compile it.

Union types: "this OR that"

function formatId(id: number | string): string {
  return `#${id}`
}

Union types describe values that can legitimately be more than one type — common for IDs that might come from different sources, or for a status field with a fixed set of allowed strings.

Generics: reusable, still typed

function firstItem<T>(list: T[]): T {
  return list[0]
}

firstItem<number>([1, 2, 3])   // returns number
firstItem<string>(['a', 'b']) // returns string

Generics let a function stay type-safe without hardcoding a specific type — the caller's input determines T, and the return type follows automatically.