Mapped types on tuples and arrays

In TypeScript 3.1, mapped object types^[1] over tuples and arrays now produce new tuples/arrays, rather than creating a new type where members like push(), pop(), and length are converted.For example:

type MapToPromise<T> = { [K in keyof T]: Promise<T[K]> };
type Coordinate = [number, number]
type PromiseCoordinate = MapToPromise<Coordinate>; // [Promise<number>, Promise<number>]

MapToPromise takes a type T, and when that type is a tuple like Coordinate, only the numeric properties are converted.In [number, number], there are two numerically named properties: 0 and 1.When given a tuple like that, MapToPromise will create a new tuple where the 0 and 1 properties are Promises of the original type.So the resulting type PromiseCoordinate ends up with the type [Promise<number>, Promise<number>].