queue-promise

queue-promise is a small, dependency-free library for promise-based queues. It will execute enqueued tasks concurrently at a given speed. When a task is being resolved or rejected, an event is emitted.

Installation

$ npm install queue-promise

Usage

With automatic start

import Queue from "queue-promise";

const queue = new Queue({
  concurrent: 1,
  interval: 2000
});

queue.on("start", () => /* … */);
queue.on("stop", () => /* … */);
queue.on("end", () => /* … */);

queue.on("resolve", data => console.log(data));
queue.on("reject", error => console.error(error));

queue.enqueue(asyncTaskA); // resolved/rejected after 0ms
queue.enqueue(asyncTaskB); // resolved/rejected after 2000ms
queue.enqueue(asyncTaskC); // resolved/rejected after 4000ms

Without automatic start

import Queue from "queue-promise";

const queue = new Queue({
  concurrent: 1,
  interval: 2000,
  start: false,
});

queue.enqueue(asyncTaskA);
queue.enqueue(asyncTaskB);
queue.enqueue(asyncTaskC);

while (queue.shouldRun) {
  // 1st iteration after 2000ms
  // 2nd iteration after 4000ms
  // 3rd iteration after 6000ms
  const data = await queue.dequeue();
}

API

new Queue(options)

Create a new Queue instance.

Option	Default	Description
concurrent	5	How many tasks should be executed in parallel
interval	500	How often should new tasks be executed (in ms)
start	true	Whether it should automatically execute new tasks as soon as they are added

public .enqueue(tasks)/.add(tasks)

Adds a new task to the queue. A task should be an async function (ES2017) or return a Promise. Throws an error if the provided task is not a valid function.

Example:

async function getRepos(user) {
  return await github.getRepos(user);
}

queue.enqueue(() => {
  return getRepos("userA");
});

queue.enqueue(async () => {
  await getRepos("userB");
});

// …equivalent to:
queue.enqueue([() => getRepos("userA"), () => getRepos("userB")]);

public .dequeue()

Executes n concurrent (based od options.concurrent) promises from the queue. Uses global Promises. Is called automatically if options.start is set to true. Emits resolve and reject events.

Example:

queue.enqueue(() => getRepos("userA"));
queue.enqueue(() => getRepos("userB"));

// If "concurrent" is set to 1, only one promise is executed on dequeue:
const userA = await queue.dequeue();
const userB = await queue.dequeue();

// If "concurrent" is set to 2, two promises are executed concurrently:
const [userA, userB] = await queue.dequeue();

Note:

.dequeue() function throttles (is executed at most once per every options.interval milliseconds).

public .on(event, callback)

Sets a callback for an event. You can set callback for those events: start, stop, resolve, reject, dequeue, end.

Example:

queue.on("dequeue", () => …);
queue.on("resolve", data => …);
queue.on("reject", error => …);
queue.on("start", () => …);
queue.on("stop", () => …);
queue.on("end", () => …);

Note:

dequeue, resolve and reject events are emitted per task. This means that even if concurrent is set to 2, 2 events will be emitted.

public .start()

Starts the queue – it will automatically dequeue tasks periodically. Emits start event.

queue.enqueue(() => getRepos("userA"));
queue.enqueue(() => getRepos("userB"));
queue.enqueue(() => getRepos("userC"));
queue.enqueue(() => getRepos("userD"));
queue.start();

// No need to call `dequeue` – you can just listen for events:
queue.on("resolve", data => …);
queue.on("reject", error => …);

public .stop()

Forces the queue to stop. New tasks will not be executed automatically even if options.start was set to true. Emits stop event.

public .clear()

Removes all tasks from the queue.

public .state

• 0: Idle state;
• 1: Running state;
• 2: Stopped state;

public .size

Size of the queue.

public .isEmpty

Whether the queue is empty, i.e. there's no tasks.

public .shouldRun

Checks whether the queue is not empty and not stopped.

Tests

$ npm test

Contributing

Development

We have prepared multiple commands to help you develop queue-promise on your own. You will need a local copy of Node.js installed on your machine. Then, install project dependencies using the following command:

$ npm install

Usage

$ npm run <command>

List of commands

Command	Description
test	Run all test:* commands described below.
test:flow	Test Flow types.
test:typescript	Test TypeScript types.
test:unit	Run unit tests.
test:lint	Run linter tests.
defs:flow	Build Flow type definitions.
defs:typescript	Build TypeScript type definitions.
clean	Clean dist directory.
build	Build package and generate type definitions.
watch	Build package in watch mode.
release	Bump package version and generate a CHANGELOG.md file.

License

queue-promise was created and developed by Bartosz Łaniewski. The full list of contributors can be found here. The package is MIT licensed.

Bug reporting

We want contributing to queue-promise to be fun, enjoyable, and educational for anyone, and everyone. Changes and improvements are more than welcome! Feel free to fork and open a pull request. We use Conventional Commits specification for commit messages. If you have found any issues, please report them here - they are being tracked on GitHub Issues.