Sometimes, we want to limit concurrency when using ES6’s Promise.all().

In this article, we’ll look at how to limit concurrency when using ES6’s Promise.all().

How to limit concurrency when using ES6’s Promise.all()?

To limit concurrency when using ES6’s Promise.all(), we can use the p-limit package.

To install it, we run

npm i p-limit

For instance, we write

const pLimit = require('p-limit');

const limit = pLimit(3);

const urls = [
  "http://www.exampleone.com/",
  "http://www.exampletwo.com/",
  "http://www.examplethree.com/",
  "http://www.examplefour.com/",
]

const promises = urls.map(url => {
  return limit(() => fetchData(url));
});

(async () => {
  const result = await Promise.all(promises);
  console.log(result);
})();

to call pLimit with 3 to return the limit function that limits the number of concurrent promises to 3.

Then we call urls.map with a callback that returns the promise that’s returned by limit.

And then call Promise.all with promises to return the results from the promises.

Conclusion

To limit concurrency when using ES6’s Promise.all(), we can use the p-limit package.