Promise.allSettled(): Never Fail a Batch of Promises
Promise.allSettled() waits for every promise in a set to finish, success or fail, without short-circuiting. Use it for independent tasks, like multiple API calls, where you need the outcome of each.
Why it exists
Before Promise.allSettled(), if you had ten independent API calls and one failed, Promise.all() would reject immediately, discarding the results of the nine successful calls. This all-or-nothing behavior is problematic for non-dependent tasks. Promise.allSettled() was created to solve this by guaranteeing you get a result for every promise.
The mental model
Think of Promise.allSettled() as a survey collector. It sends out multiple questionnaires (promises) and waits for every single one to be returned, whether it's filled out completely (fulfilled) or returned with a note explaining why (rejected). It then gives you the entire stack of responses to sift through, rather than throwing away the whole batch if one comes back incomplete.
How it works
You pass an iterable, like an array of promises, to Promise.allSettled(). It returns a single new promise that always fulfills once every input promise has settled. Its fulfillment value is an array of objects, where each object describes an outcome. Each outcome object has a 'status' property ('fulfilled' or 'rejected'). If fulfilled, it also has a 'value' property. If rejected, it has a 'reason' property. The order of the results array always matches the order of the input promises.
When to use it
Use this when you have multiple asynchronous tasks that are not dependent on one another. For example, fetching data for several unrelated widgets on a dashboard. You want to show the data for widgets that loaded successfully and an error state for those that failed, without the entire page breaking because one API call failed.
When not to use it
Do not use it when subsequent operations depend on all promises succeeding. If you need to fail an entire transaction if any single part fails, use Promise.all(). Using allSettled() in that scenario forces you to manually check for any rejections before proceeding, which is exactly the behavior Promise.all() provides automatically.
One canonical example
Imagine fetching a user's details and their list of friends from two different endpoints. One might fail, but you still want to display the other.
const p1 = api.fetchUserDetails(userId);
const p2 = api.fetchUserFriends(userId);Promise.allSettled([p1, p2]).then(results => {
const userDetailsResult = results[0];
if (userDetailsResult.status === 'fulfilled') {// render user details with userDetailsResult.value } else { // show an error for the user details section }
const friendsResult = results[1];
if (friendsResult.status === 'fulfilled') {// render friends list with friendsResult.value } }); This ensures a partial failure doesn't result in a total UI failure.
Interview question
What is the primary reason to use Promise.allSettled() over Promise.all() for multiple asynchronous tasks?
- a.To obtain a status report for every task, allowing individual handling of successes and failures.Correct
- b.To optimize performance by running all promises concurrently and discarding failed ones.
- c.To ensure the entire operation fails immediately if any single task encounters an error.
- d.To retrieve the result of the first promise that completes, whether it succeeds or fails.
Why? this is the answer
Promise.allSettled() is designed to provide an outcome for every promise in the batch, whether it fulfilled or rejected, allowing for graceful handling of partial failures. Option C describes Promise.all(), which short-circuits and rejects the entire batch if any promise fails.
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