Promise.all: Order of resolved values

by

Shortly, the order is preserved.

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

You can see this in action in the below fiddle (ES6):

// 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));
});

More Related Contents:

  1. How do I access previous promise results in a .then() chain?
  2. Wait until all promises complete even if some rejected
  3. Can I fire and forget a promise in nodejs (ES7)?
  4. How can I fetch an array of URLs with Promise.all?
  5. Is the Promise constructor callback executed asynchronously?
  6. Is Node.js native Promise.all processing in parallel or sequentially?
  7. What is the order of execution in JavaScript promises?
  8. How can I synchronously determine a JavaScript Promise’s state?
  9. Do I need to return after early resolve/reject?
  10. fetch: Reject promise with JSON error object
  11. ES6 Promise.all() error handle – Is .settle() needed? [duplicate]
  12. Resolve Javascript Promise outside the Promise constructor scope
  13. multiple, sequential fetch() Promise
  14. How to check if an object is a Promise?
  15. Should a Promise.reject message be wrapped in Error?
  16. Why can I not throw inside a Promise.catch handler?
  17. What do double brackets mean in javascript and how to access them
  18. What is the difference between “event loop queue” and “job queue”?
  19. What is the best general practice to timeout a function in promise [closed]
  20. What are asynchronous functions in JavaScript? What is “async” and “await” in JavaScript?
  21. What is the intention behind clause 2.2.4 of Promise/A+ spec?
  22. Why does the Promise object block rendering?
  23. Promises, pass additional parameters to then chain
  24. Promise and Promise.all(array) executed before array fulfilled [duplicate]
  25. Creating a (ES6) promise without starting to resolve it
  26. Node.js: When to use Promises vs Callbacks
  27. Are JavaScript forever-pending promises bad?
  28. Why does javascript ES6 Promises continue execution after a resolve?
  29. When is the body of a Promise constructor callback executed?
  30. Promise.resolve().then vs setImmediate vs nextTick

Leave a Comment