Keyof

The keyof operator takes an object type and produces a union of its keys

Like typeof, keyof is used to create new types

For any type T, keyof T is the union of public property names of T

interface Person {
  age: number;
  name: string;
}

type PersonKeys = keyof Person; // "age" | "name"

The type PersonKeys here is the same as:

type PersonKeys = "age" | "name"

If the type uses a string or number as the index, then keyof will return those types instead:

interface Arrayish = {
    [n: number]: unknown
}
type A = keyof Arrayish
// A = number

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Backlinks

• Typescript