p-queue

Promise queue with concurrency control

Useful for rate-limiting async operations. For example, when interacting with a REST API or when doing CPU/memory intensive tasks.

Install

$ npm install p-queue

Usage

Here we run only one promise at the time. For example, set concurrency to 4 to run four promises at the time.

const PQueue = require('p-queue');
const got = require('got');

const queue = new PQueue({concurrency: 1});

queue.add(() => got('sindresorhus.com')).then(() => {
    console.log('Done: sindresorhus.com');
});

queue.add(() => got('ava.li')).then(() => {
    console.log('Done: ava.li');
});

getUnicornTask().then(task => queue.add(task)).then(() => {
    console.log('Done: Unicorn task');
});

API

PQueue([options])

Returns a new queue instance.

options

Type: Object

concurrency

Type: number
Default: Infinity
Minimum: 1

Concurrency limit.

queueClass

Type: Function

Class with a enqueue and dequeue method, and a size getter. See the Custom QueueClass section.

queue

PQueue instance.

.add(fn, [options])

Returns the promise returned by calling fn.

fn

Type: Function

Promise-returning/async function.

options

Type: Object

priority

Type: number
Default: 0

Priority of operation. Operations with greater priority will be scheduled first.

.addAll(fns, [options])

Same as .add(), but accepts an array of async functions and returns a promise that resolves when all async functions are resolved.

.onEmpty()

Returns a promise that settles when the queue becomes empty.

Can be called multiple times. Useful if you for example add additional items at a later time.

.onIdle()

Returns a promise that settles when the queue becomes empty, and all promises have completed; queue.size === 0 && queue.pending === 0.

The difference with .onEmpty is that .onIdle guarantees that all work from the queue has finished. .onEmpty merely signals that the queue is empty, but it could mean that some promises haven't completed yet.

.clear()

Clear the queue.

.size

Size of the queue.

.pending

Number of pending promises.

Advanced example

A more advanced example to help you understand the flow.

const delay = require('delay');
const PQueue = require('p-queue');

const queue = new PQueue({concurrency: 1});

delay(200).then(() => {
    console.log(`8. Pending promises: ${queue.pending}`);
    //=> '8. Pending promises: 0'

    queue.add(() => Promise.resolve('🐙')).then(console.log.bind(null, '11. Resolved'));

    console.log('9. Added 🐙');

    console.log(`10. Pending promises: ${queue.pending}`);
    //=> '10. Pending promises: 1'

    queue.onIdle().then(() => {
        console.log('12. All work is done');
    });
});

queue.add(() => Promise.resolve('🦄')).then(console.log.bind(null, '5. Resolved'));
console.log('1. Added 🦄');

queue.add(() => Promise.resolve('🐴')).then(console.log.bind(null, '6. Resolved'));
console.log('2. Added 🐴');

queue.onEmpty().then(() => {
    console.log('7. Queue is empty');
});

console.log(`3. Queue size: ${queue.size}`);
//=> '3. Queue size: 1`
console.log(`4. Pending promises: ${queue.pending}`);
//=> '4. Pending promises: 1'

$ node example.js
1. Added 🦄
2. Added 🐴
3. Queue size: 1
4. Pending promises: 1
5. Resolved 🦄
6. Resolved 🐴
7. Queue is empty
8. Pending promises: 0
9. Added 🐙
10. Pending promises: 1
11. Resolved 🐙
12. All work is done

Custom QueueClass

For implementing more complex scheduling policies, you can provide a QueueClass in the options:

class QueueClass {
    constructor() {
        this._queue = [];
    }
    enqueue(run, options) {
        this._queue.push(run);
    }
    dequeue() {
        return this._queue.shift();
    }
    get size() {
        return this._queue.length;
    }
}

p-queue will call corresponding methods to put and get operations from this queue.

Related

• p-limit - Run multiple promise-returning & async functions with limited concurrency
• 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…

Created by

• Sindre Sorhus
• Vsevolod Strukchinsky

License

MIT © Sindre Sorhus