TypeScript primitive types: string, number, boolean, null, undefined

The five primitive types

TypeScript's primitive types map directly to JavaScript primitives. You annotate them with a colon after the variable name:

let username: string = "alice";
let age: number = 30;
let isActive: boolean = true;
let middleName: string | null = null;
let score: number | undefined = undefined;

Type inference

You rarely need to annotate primitives manually. TypeScript infers the type from the assigned value:

let city = "Berlin"; // inferred as string
city = 42;           // Error: Type 'number' is not assignable to type 'string'

Annotate explicitly when declaring a variable without an initial value:

let result: number;
result = fetchScore(); // assigned later

null and undefined

With strict mode enabled, null and undefined are not assignable to other types by default:

let name: string = null; // Error in strict mode
let name: string | null = null; // Correct

This forces you to handle missing values explicitly, which eliminates an entire category of runtime errors. The | syntax is a union type — covered in depth later.