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Easy rate limit with Promise.all and p-limit

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by

Rate limiting API calls to an HTTP service doesn’t have to cumbersome and too complicated.

A simple way to achieve this (and that I use all the time) is to combine Promise.all with an npm package called p-limit.

p-limit is used to run multiple promise-returning & async functions with limited concurrency.

example

In the code below I want to make concurrent calls to an api (which can be an HTTP API, library call, pretty much anything that returns a Promise), at most 5 at a time.

const pLimit = require('p-limit')
const limit = pLimit(5)

const urls = ['...', '...', ...]

return Promise.all(
  urls.map(url => limit(() => request(url)))
)

why not p-queue

p-queue would be the perfect match for a rate limited queue implementation. Altough I find it more handy, easier to read and use p-limit in combination with a simple Promise.all.

This altough applies only if you have a predefined / finite list of operations to run. If the operations are dynamic then you’ll probably go with p-queue.