Basic Types

TypeScript's scalar vocabulary: string, number, boolean, null, undefined, symbol, bigint, any, and unknown.

TypeScript adds static types on top of JavaScript values. Most annotations are optional, but the basic vocabulary helps when inference needs a hint.

Primitive annotations appear after the variable name with a colon.

const username: string = "Alice";
const age: number = 30;
const active: boolean = true;
const nothing: null = null;
const missing: undefined = undefined;
const id: symbol = Symbol("id");
const big: bigint = 9007199254740993n;

any opts out of checking. unknown is safer: you must prove the type before using the value.

let loose: any = "hello";
loose = 42;
loose.toFixed(2); // no checking
 
let raw: unknown = JSON.parse('{"x": 1}');
 
if (typeof raw === "object" && raw !== null && "x" in raw) {
  const candidate = raw as { x: unknown };
  if (typeof candidate.x === "number") {
    console.log(candidate.x);
  }
}

In production

Ban new any with lint rules and use unknown at trust boundaries: JSON.parse, API responses, queues, and plugin inputs. unknown forces one local proof before the rest of the code can treat the value as safe.

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