95 lines
3.6 KiB
Markdown
95 lines
3.6 KiB
Markdown
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# run-parallel-limit [![travis][travis-image]][travis-url] [![npm][npm-image]][npm-url] [![downloads][downloads-image]][downloads-url] [![javascript style guide][standard-image]][standard-url]
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[travis-image]: https://img.shields.io/travis/feross/run-parallel-limit/master.svg
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[travis-url]: https://travis-ci.org/feross/run-parallel-limit
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[npm-image]: https://img.shields.io/npm/v/run-parallel-limit.svg
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[npm-url]: https://npmjs.org/package/run-parallel-limit
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[downloads-image]: https://img.shields.io/npm/dm/run-parallel-limit.svg
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[downloads-url]: https://npmjs.org/package/run-parallel-limit
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[standard-image]: https://img.shields.io/badge/code_style-standard-brightgreen.svg
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[standard-url]: https://standardjs.com
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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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 [](https://saucelabs.com/u/run-parallel-limit)
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### install
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```
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npm install run-parallel-limit
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```
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### usage
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#### parallelLimit(tasks, limit, [callback])
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Run the `tasks` array of functions in parallel, with a maximum of `limit` tasks executing
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at the same time. If any of the functions pass an error to its callback, the main
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`callback` is immediately called with the value of the error. Once the `tasks` have
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completed, the results are passed to the final `callback` as an array.
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Note that the `tasks` are not executed in batches, so there is no guarantee that the first
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`limit` tasks will complete before any others are started.
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It is also possible to use an object instead of an array. Each property will be run as a
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function and the results will be passed to the final `callback` as an object instead of
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an array. This can be a more readable way of handling the results.
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##### arguments
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- `tasks` - An array or object containing functions to run. Each function is passed a
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`callback(err, result)` which it must call on completion with an error `err` (which can
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be `null`) and an optional `result` value.
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- `limit` - The maximum number of `tasks` to run at any time.
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- `callback(err, results)` - An optional callback to run once all the functions have
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completed. This function gets a results array (or object) containing all the result
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arguments passed to the task callbacks.
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##### example
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```js
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var parallelLimit = require('run-parallel-limit')
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var tasks = [
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function (callback) {
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setTimeout(function () {
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callback(null, 'one')
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}, 200)
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},
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function (callback) {
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setTimeout(function () {
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callback(null, 'two')
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}, 100)
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},
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... hundreds more tasks ...
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]
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parallelLimit(tasks, 5, function (err, results) {
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// optional callback
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// the results array will equal ['one', 'two', ...] even though
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// the second function had a shorter timeout.
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})
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```
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The above code runs with a concurrency `limit` of 5, so at most 5 tasks will be running at
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any given time.
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This module is basically equavalent to
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[`async.parallelLimit`](https://github.com/caolan/async#parallellimittasks-limit-callback),
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but it's handy to just have the one function you need instead of the kitchen sink.
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Modularity! Especially handy if you're serving to the browser and need to reduce your
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javascript bundle size.
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Works great in the browser with [browserify](http://browserify.org/)!
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### see also
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- [run-auto](https://github.com/feross/run-auto)
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- [run-parallel](https://github.com/feross/run-parallel)
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- [run-series](https://github.com/feross/run-series)
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- [run-waterfall](https://github.com/feross/run-waterfall)
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### license
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MIT. Copyright (c) [Feross Aboukhadijeh](http://feross.org).
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