11. Promise.all (Polyfill)

Implement promiseAll — a utility that behaves like native Promise.all for iterables.

Async aggregation is frequently tested in interviews. If you can implement Promise.all correctly, you’ll recognize the same coordination patterns behind Promise.race, Promise.any, and Promise.allSettled.

1) Input

• Accept an iterable of items (arrays, Sets, etc.) where each item can be:
• a Promise
• a thenable (object with then)
• a plain value

2) Resolve behavior

• Return a Promise that resolves when all items resolve.
• Resolve value must be an array of results in the same index order as the input (not completion order).

3) Reject behavior (short-circuit)

• If any item rejects, the returned Promise must reject immediately with the same reason.

4) Edge case

• Empty iterable resolves immediately with [].

Examples

// Example: Preserves index order (not completion order)
const slow = new Promise((r) => setTimeout(() => r("slow"), 20));
const fast = Promise.resolve("fast");

promiseAll([slow, fast]).then(value => {
  console.log(value); // ["slow", "fast"]
});

// Example: Treat values / thenables as resolved inputs
promiseAll([1, { then: (r) => r(2) }, Promise.resolve(3)]).then(value => {
  console.log(value); // [1, 2, 3]
});

// Example: Works with any iterable (Set example)
promiseAll(new Set([Promise.resolve(1), Promise.resolve(2)])).then(value => {
  console.log(value); // [1, 2]
});

// Example: Rejects immediately on first rejection
promiseAll([
  Promise.resolve(1),
  Promise.reject("error"),
  Promise.resolve(3)
]).catch(error => {
  console.log(error); // "error"
});