Typescript keyof

type Point = {x: number; y: number};
type P = keyof Point;
// type P = keyof Point, is the same type as “x” | “y”

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

type Mapish = {[k: string]: boolean};
type M = keyof Mapish;
// type M = string | number

Note that in this example, M 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"].