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.

For servers, you probably want a Redis-backed job queue instead.

Note that the project is feature complete. We are happy to review pull requests, but we don't plan any further development. We are also not answering email support questions.

npm install p-queue

Warning: This package is native ESM and no longer provides a CommonJS export. If your project uses CommonJS, you'll have to convert to ESM. Please don't open issues for questions regarding CommonJS / ESM.

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

import PQueue from 'p-queue';
import got from 'got';

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

(async () => {
	await queue.add(() => got('https://sindresorhus.com'));
	console.log('Done: sindresorhus.com');
})();

(async () => {
	await queue.add(() => got('https://avajs.dev'));
	console.log('Done: avajs.dev');
})();

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

Returns a new queue instance, which is an EventEmitter3 subclass.

Type: object

Type: number
Default: Infinity
Minimum: 1

Concurrency limit.

Type: number

Per-operation timeout in milliseconds. Operations fulfill once timeout elapses if they haven't already.

Type: boolean
Default: false

Whether or not a timeout is considered an exception.

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.

Type: number
Default: Infinity
Minimum: 1

The max number of runs in the given interval of time.

Type: number
Default: 0
Minimum: 0

The length of time in milliseconds before the interval count resets. Must be finite.

Type: boolean
Default: false

If true, specifies that any pending Promises, should be carried over into the next interval and counted against the intervalCap. If false, any of those pending Promises will not count towards the next intervalCap.

PQueue instance.

Adds a sync or async task to the queue.

Returns a promise with the return value of fn.

Note: If your items can potentially throw an exception, you must handle those errors from the returned Promise or they may be reported as an unhandled Promise rejection and potentially cause your process to exit immediately.

Type: Function

Promise-returning/async function. When executed, it will receive {signal} as the first argument.

Type: object

Type: number
Default: 0

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

AbortSignal for cancellation of the operation. When aborted, it will be removed from the queue and the queue.add() call will reject with an error. If the operation is already running, the signal will need to be handled by the operation itself.

import PQueue from 'p-queue';
import got, {CancelError} from 'got';

const queue = new PQueue();

const controller = new AbortController();

try {
	await queue.add(({signal}) => {
		const request = got('https://sindresorhus.com');

		signal.addEventListener('abort', () => {
			request.cancel();
		});

		try {
			return await request;
		} catch (error) {
			if (!(error instanceof CancelError)) {
				throw error;
			}
		}
	}, {signal: controller.signal});
} catch (error) {
	if (!(error instanceof DOMException)) {
		throw error;
	}
}

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 this (the instance).

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.

Returns a promise that settles when the queue size is less than the given limit: queue.size < limit.

If you want to avoid having the queue grow beyond a certain size you can await queue.onSizeLessThan() before adding a new item.

Note that this only limits the number of items waiting to start. There could still be up to concurrency jobs already running that this call does not include in its calculation.

Clear the queue.

Size of the queue, the number of queued items waiting to run.

Size of the queue, filtered by the given options.

For example, this can be used to find the number of items remaining in the queue with a specific priority level.

import PQueue from 'p-queue';

const queue = new PQueue();

queue.add(async () => '🦄', {priority: 1});
queue.add(async () => '🦄', {priority: 0});
queue.add(async () => '🦄', {priority: 1});

console.log(queue.sizeBy({priority: 1}));
//=> 2

console.log(queue.sizeBy({priority: 0}));
//=> 1

Number of running items (no longer in the queue).

Whether the queue is currently paused.

Emitted as each item is processed in the queue for the purpose of tracking progress.

import delay from 'delay';
import PQueue from 'p-queue';

const queue = new PQueue({concurrency: 2});

let count = 0;
queue.on('active', () => {
	console.log(`Working on item #${  count}.  Size: ${queue.size}  Pending: ${queue.pending}`);
});

queue.add(() => Promise.resolve());
queue.add(() => delay(2000));
queue.add(() => Promise.resolve());
queue.add(() => Promise.resolve());
queue.add(() => delay(500));

Emitted when an item completes without error.

import delay from 'delay';
import PQueue from 'p-queue';

const queue = new PQueue({concurrency: 2});

queue.on('completed', result => {
	console.log(result);
});

queue.add(() => Promise.resolve('hello, world!'));

Emitted if an item throws an error.

import delay from 'delay';
import PQueue from 'p-queue';

const queue = new PQueue({concurrency: 2});

queue.on('error', error => {
	console.error(error);
});

queue.add(() => Promise.reject(new Error('error')));

Emitted every time the queue becomes empty.

Useful if you for example add additional items at a later time.

Emitted every time the queue becomes empty and all promises have completed; queue.size === 0 && queue.pending === 0.

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

import delay from 'delay';
import PQueue from 'p-queue';

const queue = new PQueue();

queue.on('idle', () => {
	console.log(`Queue is idle.  Size: ${queue.size}  Pending: ${queue.pending}`);
});

const job1 = queue.add(() => delay(2000));
const job2 = queue.add(() => delay(500));

await job1;
await job2;
// => 'Queue is idle.  Size: 0  Pending: 0'

await queue.add(() => delay(600));
// => 'Queue is idle.  Size: 0  Pending: 0'

The idle event is emitted every time the queue reaches an idle state. On the other hand, the promise the onIdle() function returns resolves once the queue becomes idle instead of every time the queue is idle.

Emitted every time the add method is called and the number of pending or queued tasks is increased.

Emitted every time a task is completed and the number of pending or queued tasks is decreased. This is emitted regardless of whether the task completed normally or with an error.

import delay from 'delay';
import PQueue from 'p-queue';

const queue = new PQueue();

queue.on('add', () => {
	console.log(`Task is added.  Size: ${queue.size}  Pending: ${queue.pending}`);
});

queue.on('next', () => {
	console.log(`Task is completed.  Size: ${queue.size}  Pending: ${queue.pending}`);
});

const job1 = queue.add(() => delay(2000));
const job2 = queue.add(() => delay(500));

await job1;
await job2;
//=> 'Task is added.  Size: 0  Pending: 1'
//=> 'Task is added.  Size: 0  Pending: 2'

await queue.add(() => delay(600));
//=> 'Task is completed.  Size: 0  Pending: 1'
//=> 'Task is completed.  Size: 0  Pending: 0'

A more advanced example to help you understand the flow.

import delay from 'delay';
import PQueue from 'p-queue';

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

(async () => {
	await delay(200);

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

	(async () => {
		await queue.add(async () => '🐙');
		console.log('11. Resolved')
	})();

	console.log('9. Added 🐙');

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

	await queue.onIdle();
	console.log('12. All work is done');
})();

(async () => {
	await queue.add(async () => '🦄');
	console.log('5. Resolved')
})();
console.log('1. Added 🦄');

(async () => {
	await queue.add(async () => '🐴');
	console.log('6. Resolved')
})();
console.log('2. Added 🐴');

(async () => {
	await queue.onEmpty();
	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:

import PQueue from 'p-queue';

class QueueClass {
	constructor() {
		this._queue = [];
	}

	enqueue(run, options) {
		this._queue.push(run);
	}

	dequeue() {
		return this._queue.shift();
	}

	get size() {
		return this._queue.length;
	}

	filter(options) {
		return this._queue;
	}
}

const queue = new PQueue({queueClass: QueueClass});

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

They are just different constraints. The concurrency option limits how many things run at the same time. The intervalCap option limits how many things run in total during the interval (over time).

• Sindre Sorhus
• Richie Bendall

• 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…