TypeScript Basic Types

Overview

TypeScript adds static types to JavaScript. Types are checked at compile time and erased at runtime. Start with primitives and collections, then layer functions and objects as your codebase grows.

Syntax / Usage

let name: string = 'Ada'
let age: number = 36
let active: boolean = true

const ids: number[] = [1, 2, 3]
const tags: Array<string> = ['ts', 'web']

// Tuple — fixed length and types
const point: [number, number] = [10, 20]

// any disables checking (avoid when possible)
let legacy: any = 'anything'

// unknown — safer than any; narrow before use
let input: unknown = getInput()
if (typeof input === 'string') {
  console.log(input.toUpperCase())
}

// void and never
function log(msg: string): void {
  console.log(msg)
}

Examples

Function with typed parameters and return:

function formatPrice(amount: number, currency = 'USD'): string {
  return new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency,
  }).format(amount)
}

Readonly arrays:

const ROLES: readonly string[] = ['admin', 'user']
// ROLES.push('guest') — error

Common Mistakes

• Overusing any, defeating TypeScript's purpose
• Assuming runtime validation—types disappear after compilation
• Using object when a specific shape is known—prefer interfaces
• Confusing null and undefined without strictNullChecks

See Also

interfaces unions type-guards generics