TypeScript 7 reference · Type Operators and Keywords

TypeScript as const Keyword

Make a value deeply read-only with literal types: strings stay 'admin' instead of string, arrays become readonly tuples.

TypeScript 7 Since 3.4

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Syntax

value as const

Examples

Literal types from arraysTry it yourself

const plain = ["GET", "POST"];          // string[]
const exact = ["GET", "POST"] as const; // readonly ["GET", "POST"]
type Method = (typeof exact)[number];   // "GET" | "POST"
const m: Method = "GET";
console.log(plain.length, exact.length, m);

Output

2 2 GET

Readonly objectsTry it yourself

const settings = { mode: "dark", size: 14 } as const;
// @ts-expect-error: as const makes properties readonly
settings.mode = "light";
console.log(settings);

Output

{ mode: 'light', size: 14 }

Version history

Version	Change
TypeScript 3.4	Added

satisfies · Check that a value matches a type without widening it, so you keep the exact inferred type and still get errors.
Readonly<Type> · Make every property of a type read-only, so the compiler rejects assignments to them after creation.
typeof (type query) · In a type position, typeof takes the type of a value or function, so you don't have to write it twice.
TypeScript keyof, typeof and Indexed Access · tutorial

Syntax

Examples

Version history

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