queue-promise

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

Installation

$ npm install queue-promise

Usage

import Queue from "queue-promise";

const queue = new Queue({
  // How many tasks should be executed in parallel (defaults to `5`):
  concurrent: 1,
  // How often should new tasks be executed (in ms – defaults to `500`):
  interval: 2000,
  // If should resolve new tasks automatically when added (defaults to `true`):
  start: true
});

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 0s
queue.enqueue(asyncTaskB); // resolved/rejected after 2s
queue.enqueue(asyncTaskC); // resolved/rejected after 4s
queue.enqueue(asyncTaskD); // resolved/rejected after 6s

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 resolve 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(getRepos("userA"));
queue.enqueue(getRepos("userB"));

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

public .dequeue()

Manually resolves 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 resolved on dequeue:
const userA = await queue.dequeue();
const userB = await queue.dequeue();

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

public .on(event, callback)

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

Example:

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

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 resolved automatically even if options.start was set to true. Emits stop event.

public .clear()

Removes all tasks from the queue.

public .started

Whether the queue has been started or not.

public .stopped

Whether the queue has been forced to stop.

public .isEmpty

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

Tests

$ npm test