Basic Types and Type Annotations

TypeScript provides a rich set of built-in types that map closely to JavaScript's runtime values. Understanding these foundational types is essential before moving on to more advanced features. This lesson covers every primitive type, arrays, tuples, and the special types that TypeScript adds on top of JavaScript.

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Type Annotations

A type annotation tells the compiler what type a variable, parameter, or return value should be. You write a colon followed by the type after the identifier:

let username: string = 'Alice';
let age: number = 30;
let isActive: boolean = true;

TypeScript can often infer the type from the assigned value, so explicit annotations are optional in many cases. However, they are invaluable for function signatures and complex objects.

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Primitive Types

string

Represents textual data. Template literals are fully supported:

let greeting: string = 'Hello';
let message: string = `Welcome, ${greeting}`;

number

All numbers in TypeScript (and JavaScript) are floating-point values. There is no separate integer type:

let count: number = 42;
let price: number = 9.99;
let hex: number = 0xff;
let binary: number = 0b1010;

boolean