TypeScript:keyof
lastmod 2023-06-09created 2022-12-154 edits
The keyof operator takes an object type and produces a string or numeric literal union of its keys.
Simple example
The type P below is the same type as "x" | "y".
If the type has a string or number index signature, keyof returns those types instead.
type Arrayish = { [n: number]: unknown };
type A = keyof Arrayish;
type Mapish = { [k: string]: boolean };
type M = keyof Mapish;
Note that in the example above, M is string | number. - This is because JavaScript object keys are always coerced to a string, so obj[0] is the same as obj["0"].
keyof types become especially useful when combined with mapped types, which we'll learn more about later.