Basic Types

TypeScript covers all of JavaScript's basic types — string, number, boolean, arrays, and a few TypeScript-specific ones — with a syntax for writing them explicitly, and a compiler that's often smart enough to figure them out without being told.

The core primitives

let username: string = "jordan";
let age: number = 29;
let isAdmin: boolean = false;

console.log(username, age, isAdmin);

jordan 29 false

Unlike C or Java, TypeScript only has one numeric type, number, covering both integers and decimals — there's no separate int or float.

Type inference

Writing out : string everywhere gets repetitive, and usually isn't necessary — TypeScript infers a variable's type from whatever it's initialized with:

let city = "Lisbon";
city = 42;

inference.ts:2:1 - error TS2322: Type 'number' is not
assignable to type 'string'.

Even without an annotation, city is locked in as a string the moment it's initialized with one — TypeScript inferred the type from the value and enforces it from then on.

Arrays and any

let scores: number[] = [88, 92, 79];
let names: Array<string> = ["Ana", "Ben"];

scores.push("nope");

arrays.ts:4:13 - error TS2345: Argument of type 'string' is
not assignable to parameter of type 'number'.

number[] and Array<number> are two ways of writing the exact same type — an array whose every element must be a number. Either syntax works; most codebases just pick one style and stay consistent.