Basic Types

Syntax

let name: string;
let age: number;
let isActive: boolean;

Examples

let name: string = "Alice";
let age: number = 25;
let isActive: boolean = true;

// TypeScript catches mismatches immediately
// age = "twenty-five"; // Error: Type 'string' is not assignable to type 'number'

let id: string | number = 101; // union type, can be either

let flexible: any = "hello";
flexible.toUpperCase(); // allowed, no error - any disables checking entirely
flexible = 42;           // also allowed, any accepts anything

let safer: unknown = "hello";
// safer.toUpperCase(); // Error - must narrow the type first

if (typeof safer === "string") {
  safer.toUpperCase(); // OK now - TypeScript knows it is a string here
}

function logMessage(message: string): void {
  console.log(message); // no return statement - return type is void
}

function throwError(message: string): never {
  throw new Error(message); // never actually returns - always throws
}

function infiniteLoop(): never {
  while (true) { /* never exits */ }
}

Best practices

• Prefer unknown over any whenever possible - it forces you to check a value's type before using it, catching bugs any would silently allow
• Let TypeScript infer types where it reasonably can (const age = 25) rather than annotating every single variable - annotate mainly function parameters and return types
• Use never for functions that always throw or never return, which helps TypeScript correctly narrow types in the code that calls them
• Avoid any as a habit - it is sometimes necessary for gradual migration or truly dynamic data, but it opts out of type safety entirely wherever it is used

At a glance

Purpose

Static types for JavaScript

File extension

.ts · .tsx

Runs in

Compiled to JavaScript

Usually used with

JavaScript and its ecosystem

Specifications & further reading

• TypeScript Handbook

Related TypeScript documentation