[JavaScript] Handling multiple promises

When you have multiple promises you want to run in parallel, you can use Promise.all() or Promise.allSettled() to process them. See the comments and the note below for the key difference.

const promise1 = getPromise1();
const promise2 = getPromise2();

// `resultsAll` will be an array of the resolved values of promise1 and promise2.
const resultsAll = await Promise.all([promise1, promise2]);

// `resultsAllSettled` will be an array of objects that describe the result of each promise,
// including a `status` of 'fulfilled' or 'rejected', and a `value` or `reason`.
const resultsAllSettled = await Promise.allSettled([promise1, promise2]);

Note

From the docs:

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.

Docs

• Promise.all()
• Promise.allSettled()

Here is an example of using Promise.allSettled() and collecting up the errors:

const promises = [promise1, promise2];
const results = await Promise.allSettled(promises);
const errors = results.filter(result => result.status === 'rejected');

Tip

You can read about more promise concurrency methods: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise#promise_concurrency

Iterate over a list of items and run promises in parallel or serial

Sometimes you’ll have a list of items, and you want to run a promise for each item in parallel. For example, a list of IDs that you want to fetch data for from an API. You could map the items into promises first and then use Promise.all() to run them all in parallel.

const ids = [1, 2, 3, 4, 5];
const promises = ids.map(id => getData(id));
const results = await Promise.all(promises);

To process these promise in serial (i.e. guaranteed one by one), you can use for...of to iterate over the items and run the promises:

const ids = [1, 2, 3, 4, 5];
for (const id of ids) {
  await getData(id);
}

Caution

Never use .forEach to iterate over items and await promises. This will not work as expected because .forEach was not designed for asynchronous code — it does not wait for the promises to resolve (thus not guaranteeing serial behaviour), and does not catch errors. See https://gist.github.com/joeytwiddle/37d2085425c049629b80956d3c618971 for a detailed explanation.