Order of resolved values in Promise.all()

Shortly, Promise.all() preserves the order between its input and output.

Following the spec, Promise.all(iterable) takes an iterable as a parameter and internally calls PerformPromiseAll(iterator, constructor, resultCapability) with it, where the latter loops over the iterable using IteratorStep(iterator).

Resolving is implemented via Promise.all() Resolve where each resolved promise has an internal [[Index]] slot, which marks the index of the promise in the original input.

All this means the output is strictly ordered given the iterable you pass to Promise.all() is strictly ordered (for example, an array):

// Used to display results
const write = msg => {
  document.body.appendChild(document.createElement('div')).innerHTML = msg;

// Different speed async operations
const slow = new Promise(resolve => {
  setTimeout(resolve, 200, 'slow');
const instant = 'instant';
const quick = new Promise(resolve => {
  setTimeout(resolve, 50, 'quick');

// The order is preserved regardless of what resolved first
Promise.all([slow, instant, quick]).then(responses => {
  responses.map(response => write(response));