Typescript function to await all promised object properties

promise-all-keys.ts

type Unpacked<T> =
	T extends (infer U)[]
		? U
		: T extends (...args: any[]) => infer U
			? U
			: T extends Promise<infer U>
				? U
				: T

type UnpackedObject<T> = {
	[P in keyof T]: Unpacked<T[P]>
}
  
interface ObjInput {
	[key: string]: Promise<any>
}

export async function promiseAllKeys<T extends ObjInput>(obj: T): Promise<UnpackedObject<T>> {
	const operations = Object.keys(obj).map(async key => [key, await obj[key]])
	const newObject = {}
	for (const [key, value] of await Promise.all(operations))
		newObject[key] = value
	return <UnpackedObject<T>>newObject
}

There's a new Awaited utility type starting from TS version 4.5

const all = async <T extends object>(promises: T) => {
  const result = {} as {
    [P in keyof T]: Awaited<T[P]>
  }
  for (const [key, promise] of Object.entries(promises)) {
    result[key as keyof T] = await promise
  }
  return result
}

Here's a version that allows parallelism, and uses the ES2019 fromEntries.

const liftKeysP = async <T extends object>(obj: T) => {
  const ops = await Promise.all(Object.entries(obj).map(async ([k, p]) => [k, await p]))
  return Object.fromEntries(ops) as {
    [P in keyof T]: Awaited<T[P]>
  }
}

@Nicholaiii yep that's super good, that version is best 👍