p-limit

What is p-limit?

The p-limit npm package is a utility that allows you to limit the number of promises that are running at the same time. It is useful for controlling concurrency when you have operations that can be run in parallel but you want to limit the number of these operations due to resource constraints.

What are p-limit's main functionalities?

Concurrency Limiting

This feature allows you to create a limit for how many promises are allowed to run at once. In the code sample, the limit is set to 1, meaning that `doSomething` and `doAnotherThing` will not run at the same time.

const pLimit = require('p-limit');
const limit = pLimit(1);

async function doSomething() {}

async function doAnotherThing() {}

// Only one promise will run at once
const result1 = limit(() => doSomething());
const result2 = limit(() => doAnotherThing());

Queueing

This feature demonstrates how additional promises are queued when the limit is reached. In this example, only two promises will run concurrently, and the rest will wait in the queue.

const pLimit = require('p-limit');
const limit = pLimit(2);

const input = [
  limit(() => fetchSomething('foo')),
  limit(() => fetchSomething('bar')),
  limit(() => doSomethingElse()),
];

// Only two promises will run at once, the rest will be queued
Promise.all(input).then(results => {
  console.log(results);
});

Other packages similar to p-limit

bottleneck

Bottleneck is a powerful rate limiter that allows you to limit the number of calls to a function. It can be used to throttle requests to an API or any other operation that needs to be rate-limited. It is more feature-rich than p-limit, providing priorities, clustering support, and more configuration options.

async

Async provides a collection of utilities for working with asynchronous JavaScript. While it offers functions like `async.parallelLimit` which can limit the number of asynchronous operations running in parallel, it is a more comprehensive toolkit for asynchronous control flow, including series, waterfall, and eachOf methods.

p-queue

P-Queue is a promise queue with adjustable concurrency that supports priorities, timeouts, and pausing. It is similar to p-limit but offers a higher level of abstraction with a queue system, making it suitable for more complex scenarios where you need to manage the order and priority of tasks.

README

p-limit

Run multiple promise-returning & async functions with limited concurrency

Works in Node.js and browsers.

Install

npm install p-limit

Usage

import pLimit from 'p-limit';

const limit = pLimit(1);

const input = [
	limit(() => fetchSomething('foo')),
	limit(() => fetchSomething('bar')),
	limit(() => doSomething())
];

// Only one promise is run at once
const result = await Promise.all(input);
console.log(result);

API

pLimit(concurrency) ^{default export}

Returns a limit function.

concurrency

Type: number
Minimum: 1

Concurrency limit.

limit(fn, ...args)

Returns the promise returned by calling fn(...args).

fn

Type: Function

Promise-returning/async function.

args

Any arguments to pass through to fn.

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.

limit.activeCount

The number of promises that are currently running.

limit.pendingCount

The number of promises that are waiting to run (i.e. their internal fn was not called yet).

limit.clearQueue()

Discard pending promises that are waiting to run.

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.

Note: This does not cancel promises that are already running.

limit.concurrency

Get or set the concurrency limit.

limitFunction(fn, options) ^{named export}

Returns a function with limited concurrency.

The returned function manages its own concurrent executions, allowing you to call it multiple times without exceeding the specified concurrency limit.

Ideal for scenarios where you need to control the number of simultaneous executions of a single function, rather than managing concurrency across multiple functions.

import {limitFunction} from 'p-limit';

const limitedFunction = limitFunction(async () => {
	return doSomething();
}, {concurrency: 1});

const input = Array.from({length: 10}, limitedFunction);

// Only one promise is run at once.
await Promise.all(input);

fn

Type: Function

Promise-returning/async function.

options

Type: object

concurrency

Type: number
Minimum: 1

Concurrency limit.

FAQ

How is this different from the p-queue package?

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.

Related

• p-throttle - Throttle promise-returning & async functions
• p-debounce - Debounce promise-returning & async functions
• p-all - Run promise-returning & async functions concurrently with optional limited concurrency
• More…