jsschools

keyof

Produces a union of the property key names of a given type.

Since TS 2.1 Spec ↗

Syntax

keyof Type

Examples

interface User { id: number; name: string }
type UserKeys = keyof User; // 'id' | 'name'

function get<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

get({ a: 1, b: 'x' }, 'b'); // string

Notes

The `keyof` operator yields the literal union of an object type's keys. For an index signature `{ [k: string]: V }` it yields `string | number`. Commonly paired with indexed access types and generic constraints for type-safe property access.

See also