Promise queue with concurrency control

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

$ npm install p-queue

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');
});

Returns a new queue instance.

Type: Object

Type: number
Default: Infinity
Minimum: 1

Concurrency limit.

Type: boolean
Default: true

Whether queue tasks within concurrency limit, are auto-executed as soon as they're added.

Type: Function

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

PQueue instance.

Adds a sync or async task to the queue. Always returns a promise.

Type: Function

Promise-returning/async function.

Type: Object

Type: number
Default: 0

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

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

Put queue execution on hold.

Start (or resume) executing enqueued tasks within concurrency limit. No need to call this if queue is not paused (via options.autoStart = false or by .pause() method.)

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.

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 the queue.

Size of the queue.

Number of pending promises.

Whether the queue is currently paused.

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

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.

• 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…

• Sindre Sorhus
• Vsevolod Strukchinsky

MIT © Sindre Sorhus