Run an array of functions in parallel, but limit the number of tasks executing at the same time
Last updated 2 years ago by feross .
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$ cnpm install run-parallel-limit 
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Run an array of functions in parallel, but limit the number of tasks executing at the same time

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npm install run-parallel-limit


parallelLimit(tasks, limit, [callback])

Run the tasks array of functions in parallel, with a maximum of limit tasks executing at the same time. If any of the functions pass an error to its callback, the main callback is immediately called with the value of the error. Once the tasks have completed, the results are passed to the final callback as an array.

Note that the tasks are not executed in batches, so there is no guarantee that the first limit tasks will complete before any others are started.

It is also possible to use an object instead of an array. Each property will be run as a function and the results will be passed to the final callback as an object instead of an array. This can be a more readable way of handling the results.

  • tasks - An array or object containing functions to run. Each function is passed a callback(err, result) which it must call on completion with an error err (which can be null) and an optional result value.
  • limit - The maximum number of tasks to run at any time.
  • callback(err, results) - An optional callback to run once all the functions have completed. This function gets a results array (or object) containing all the result arguments passed to the task callbacks.
var parallelLimit = require('run-parallel-limit')

var tasks = [
  function (callback) {
    setTimeout(function () {
      callback(null, 'one')
    }, 200)
  function (callback) {
    setTimeout(function () {
      callback(null, 'two')
    }, 100)
  ... hundreds more tasks ...

parallelLimit(tasks, 5, function (err, results) {
  // optional callback
  // the results array will equal ['one', 'two', ...] even though
  // the second function had a shorter timeout.

The above code runs with a concurrency limit of 5, so at most 5 tasks will be running at any given time.

This module is basically equavalent to async.parallelLimit, but it's handy to just have the one function you need instead of the kitchen sink. Modularity! Especially handy if you're serving to the browser and need to reduce your javascript bundle size.

Works great in the browser with browserify!

see also


MIT. Copyright (c) Feross Aboukhadijeh.

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