1 | # p-limit [![Build Status](https://travis-ci.org/sindresorhus/p-limit.svg?branch=master)](https://travis-ci.org/sindresorhus/p-limit)
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2 |
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3 | > Run multiple promise-returning & async functions with limited concurrency
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4 |
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5 | ## Install
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6 |
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7 | ```
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8 | $ npm install p-limit
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9 | ```
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10 |
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11 | ## Usage
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12 |
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13 | ```js
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14 | const pLimit = require('p-limit');
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15 |
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16 | const limit = pLimit(1);
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17 |
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18 | const input = [
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19 | limit(() => fetchSomething('foo')),
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20 | limit(() => fetchSomething('bar')),
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21 | limit(() => doSomething())
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22 | ];
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23 |
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24 | (async () => {
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25 | // Only one promise is run at once
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26 | const result = await Promise.all(input);
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27 | console.log(result);
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28 | })();
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29 | ```
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30 |
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31 | ## API
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32 |
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33 | ### pLimit(concurrency)
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34 |
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35 | Returns a `limit` function.
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36 |
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37 | #### concurrency
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38 |
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39 | Type: `number`\
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40 | Minimum: `1`\
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41 | Default: `Infinity`
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42 |
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43 | Concurrency limit.
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44 |
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45 | ### limit(fn, ...args)
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46 |
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47 | Returns the promise returned by calling `fn(...args)`.
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48 |
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49 | #### fn
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50 |
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51 | Type: `Function`
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52 |
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53 | Promise-returning/async function.
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54 |
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55 | #### args
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56 |
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57 | Any arguments to pass through to `fn`.
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58 |
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59 | Support for passing arguments on to the `fn` is provided in order to be able to avoid creating unnecessary closures. You probably don't need this optimization unless you're pushing a *lot* of functions.
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60 |
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61 | ### limit.activeCount
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62 |
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63 | The number of promises that are currently running.
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64 |
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65 | ### limit.pendingCount
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66 |
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67 | The number of promises that are waiting to run (i.e. their internal `fn` was not called yet).
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68 |
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69 | ### limit.clearQueue()
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70 |
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71 | Discard pending promises that are waiting to run.
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72 |
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73 | This might be useful if you want to teardown the queue at the end of your program's lifecycle or discard any function calls referencing an intermediary state of your app.
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74 |
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75 | Note: This does not cancel promises that are already running.
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76 |
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77 | ## FAQ
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78 |
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79 | ### How is this different from the [`p-queue`](https://github.com/sindresorhus/p-queue) package?
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80 |
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81 | This package is only about limiting the number of concurrent executions, while `p-queue` is a fully featured queue implementation with lots of different options, introspection, and ability to pause the queue.
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82 |
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83 | ## Related
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84 |
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85 | - [p-queue](https://github.com/sindresorhus/p-queue) - Promise queue with concurrency control
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86 | - [p-throttle](https://github.com/sindresorhus/p-throttle) - Throttle promise-returning & async functions
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87 | - [p-debounce](https://github.com/sindresorhus/p-debounce) - Debounce promise-returning & async functions
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88 | - [p-all](https://github.com/sindresorhus/p-all) - Run promise-returning & async functions concurrently with optional limited concurrency
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89 | - [More…](https://github.com/sindresorhus/promise-fun)
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90 |
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91 | ---
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92 |
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93 | <div align="center">
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94 | <b>
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95 | <a href="https://tidelift.com/subscription/pkg/npm-p-limit?utm_source=npm-p-limit&utm_medium=referral&utm_campaign=readme">Get professional support for this package with a Tidelift subscription</a>
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96 | </b>
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97 | <br>
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98 | <sub>
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99 | Tidelift helps make open source sustainable for maintainers while giving companies<br>assurances about security, maintenance, and licensing for their dependencies.
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100 | </sub>
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101 | </div>
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