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The keyof type operator

The keyof operator takes an object type and produces a string or numeric literal union of its keys.

type Point = {
  x: number;
  y: number;
};

type P = keyof Point;

// type P = 'x' | 'y'

let p1: P = "x";
let p2: P = "y";

let p3: P = 1; // Type '1' is not assignable to type 'keyof Point'.
type Person = {
  name: string;
};

type PersonKey = keyof Person;
// type PersonKey = 'name'

If the type has a string or number index signature, keyof will return those types instead:

type Person = { [key: string]: string | number };

type P = keyof Person;
// type P = string | number

P is string | number ---- this is because JavaScript Object keys are always coerced to a string, so obj[0] is always the same as obj['0'].

type Arrayish = { [key: number]: unknown };

type NumberType = keyof Arrayish;
// type NumberType = number