ts-async-queue

A simple async-await task queue written in TypeScript

npm i -S ts-async-queue

• Installation options
• API

What is it?

This is just a simple library for fail-safe async task execution, written in TypeScript. It utilizes OOP principles and TypeScript for painless management of complex task queues and easy extendability.

Unlike many similar libraries for working with async queues, this one behaves more like a "task-player" with start, pause, resume and stop functions. If you need execution concurrency or complex priority management - look elsewhere. Here you can simply enqueue, dequeue, start, stop, pause and resume in just 2KB of minified code. So if you need simplicity and small package - this is your choice.

Installation options

ts-async-queue comes with a couple of installation options:

format-name	exports	description
es5	named	Contains an es5-compatible version of ts-async-queue
es	named	ES6+-compatible version
esnext	named	ES7+ version with non-transpiled async-awaits
iife	tsQueue object	Creates a global variable tsQueue containing the exports. window-only.
umd	tsQueue object	Same as iife, but also works for node.

The dist folder contains multiple files with names in a following format: /dist/ts-async-queue.{format-name}.js, where {format-name} is a format name from the table.

Named imports

// ES5/CommonJS
const { TaskQueue, QueueError } = require('ts-async-queue');

// If the first one does not work in your environment:
const { TaskQueue, QueueError } = require('ts-async-queue/dist/ts-async-queue.es5');

// ES6+/TypeScript
import { TaskQueue, QueueError } from 'ts-async-queue';
import { TaskQueue, QueueError } from 'ts-async-queue/dist/ts-async-queue.es'; // ES6-only
import { TaskQueue, QueueError } from 'ts-async-queue/dist/ts-async-queue.esnext'; // ES7+ only

Script imports

<!-- IIFE -->
<script src="https://unpkg.com/ts-async-queue"></script>

<!-- For specific versions of the library -->
<script src="https://unpkg.com/ts-async-queue/dist/ts-async-queue.{export-version}.js"></script>

Library exports

ts-async-queue has 2 named exports: TaskQueue and QueueError:

const { TaskQueue, QueueError } = require('ts-async-queue');

import { TaskQueue, QueueError } from 'ts-async-queue';

TaskQueue

Manages a queue of async tasks

See type declaration for more details

Initilize queue and manage tasks:

// Task is a simple function that recieves 0 arguments and returns a promise:
const task1 = () => Promise.resolve('task1');
const task2 = () => Promise.resolve('task2');
const task3 = () => Promise.resolve('task3');

// Initialize an empty task queue:
const queue = new TaskQueue();

// and enqueue tasks of your choice (one or more):
queue.enqueue(task1, task2, task3); // -> queue contains [task1, task2, task3]

// or dequeue them

// by index
queue.dequeue(1); // -> returns task2; queue contains [task1, task3]

// by task reference
queue.dequeue(task1); // -> returns task1; queue contains [task3]

// or simply pop the last one
queue.dequeue(); // dequeues the last task in; queue is empty now.
queue.pop(); // same as previous

// Or initialize a task queue with default tasks:
const queueWithElements = new TaskQueue([
  task1,
  task2,
  task3
]);

// Clear all tasks at once
queueWithElements.clear(); // -> queue is empty now ([]).

queue.start()

Starts task queue execution. Returns currenlty executed queue if execution already started.

const queue = new TaskQueue([
  task1,
  task2,
  task3
]);


/** Start the queue with
 * TaskQueue.start():
 */

// You can start a queue by calling the `start` function that returns a promise:
const endOfQueueExecution = queue.start();

// endOfQueueExecution is resolved as soon as the queue has finished executing all tasks,
// it always resolves to the array of task results in order of their execution:
endOfQueueExecution.then(results => {
  // If you only need only the last result, just do
  const result = results[results.length - 1];

  console.log(result); // -> task3
  console.log(results); // -> ['task1', 'task2', 'task3']
});

// If you don't care about results, you can also simply await the `start()`:
await queue.start(); // does the exact same procedure as the previous time
// .. do here whatever you want to do after the tasks stop executing

// If one execution is not enough, you can always restart:
queue.start();

queue.stop()

Stops queue execution and clears results.

/** Stop the queue with
 * TaskQueue.stop():
 */

// You can stop the queue execution at any task:
const stopped = queue.stop();

// The `stopped` promise resolves as soon as the queue stops executing:
stopped.then(results => {
  // Here, `results` contains the results of whatever tasks managed to finish their execution

  // If we try to stop the queue again, it will return the same result:
  queue.stop()
    .then(res => console.log(res)); // `res` is equal to `results`
});

queue.pause()

Pauses the queue's execution flow after a nearest task is completed. Returns a promise that resolves as soon as the queue is paused

queue.start();

/** Pause the queue with
 * TaskQueue.pause():
 */

// Pause resolves to results of tasks executed before the execution has been paused
const results = await queue.pause()
const amountOfTasksExecuted = results.length;

queue.resume()

Resumes a previously paused queue. Returns a promise that resolves as soon as the queue is completed

/** Resume the queue with
 * TaskQueue.resume():
 */

const results = await queue.resume();

console.log(results); // -> ['task1', 'task2', 'task3']

Instance properties

name	type	description
isRunning	boolean	true if the queue is currently executing
length	number	Amount of tasks in the queue
last	Task	The last task added to the queue
peek()	Task	A method alias for last
lastResults	Array	An array of results captured from the last queue execution
currentTaskIndex	number	A task index at which the queue is currently running
currentRunningTask	Task	A task which is currently running in the queue

QueueError

A simple error class that is raised if the queue encountered an error in a currently running task

Properties:

name	type	description
message	string	An error message text
queue	TaskQueue	The queue instance error is raised from
data	any	Internal error data
stack	string	Error stack trace
failedTask	Task	The task at which the queue has failed
failedTaskIndex	number	An index of the task at which the queue has failed

Usage example

const queue1 = new TaskQueue([
  () => Promise.resolve('task1'),
  () => Promise.resolve('task2')
]);

const queue2 = new TaskQueue([
  () => Promise.reject('task1') // This one rejects the promise, causing an exception to be thrown,
  () => Promise.resolve('task2')
]);

try {
  await queue1.start();
  await queue2.start();
} catch (e) {
  console.log(e.queue === queue2); // -> true

  console.log(e); // -> Queue paused at task #1 due to error in handler () => Promise.reject('task1')
}