Skip to content
Onsite
17 / 28

Typed PromiseAll

MediumP2Coding

Declare the type of a function PromiseAll(values) that behaves like Promise.all: it takes an array or tuple whose items may be plain values or promises, and returns a promise of a tuple of the awaited values in the same order.

A tuple literal keeps its length and element types; a non-tuple array of number | Promise<number> gives number[].

Tests your type must pass

const promiseAllTest1 = PromiseAll([1, 2, 3] as const)
const promiseAllTest2 = PromiseAll([1, 2, Promise.resolve(3)] as const)
const promiseAllTest3 = PromiseAll([1, 2, Promise.resolve(3)])
const promiseAllTest4 = PromiseAll<Array<number | Promise<number>>>([1, 2, 3])
const promiseAllTest5 = PromiseAll([] as const)

type cases = [
  Expect<Equal<typeof promiseAllTest1, Promise<[1, 2, 3]>>>,
  Expect<Equal<typeof promiseAllTest2, Promise<[1, 2, number]>>>,
  Expect<Equal<typeof promiseAllTest3, Promise<[number, number, number]>>>,
  Expect<Equal<typeof promiseAllTest4, Promise<number[]>>>,
  Expect<Equal<typeof promiseAllTest5, Promise<[]>>>,
]

Hints

TypeScript
Run and Submit type-check your solution against the tests.