Promise.all()

The Promise.all() method takes an iterable of promises as input and returns a single Promise. This returned promise fulfills when all of the input's promises fulfill (including when an empty iterable is passed), with an array of the fulfillment values. It rejects when any of the input's promises rejects, with this first rejection reason.

Syntax

Promise.all(iterable)

Parameters

• iterable
  • : An iterable (such as an Array of promises.

Return value

A Promise that is:

• Already fulfilled, if the iterable passed is empty.
• Asynchronously fulfilled, when all the promises in the given iterable fulfill. The fulfillment value is an array of fulfillment values, in the order of the promises passed, regardless of completion order. If the iterable passed is non-empty but contains no pending promises, the returned promise is still asynchronously (instead of synchronously) fulfilled.
• Asynchronously rejected, when any of the promises in the given iterable rejects. The rejection reason is the rejection reason of the first promise that was rejected.

Description

The Promise.all() method is one of the promise concurrency methods. It can be useful for aggregating the results of multiple promises. It is typically used when there are multiple related asynchronous tasks that the overall code relies on to work successfully — all of whom we want to fulfill before the code execution continues.

Promise.all() will reject immediately upon any of the input promises rejecting. In comparison, the promise returned by Promise.allSettled() will wait for all input promises to complete, regardless of whether or not one rejects. Use allSettled() if you need the final result of every promise in the input iterable.