Prioritized Task Scheduling API

Note: This feature is available in Web Workers

The Prioritized Task Scheduling API provides a standardized way to prioritize all tasks belonging to an application, whether they defined in a website developer's code, or in third party libraries and frameworks.

The task priorities are very coarse-grained, and are based around whether tasks block user interaction, or otherwise impact the user experience, or can run in the background. Developers and frameworks may implement more fine-grained prioritization schemes within the broad categories define by the API

The API is promise-based and supports the ability to set and change task priorities, to delay tasks being added to the scheduler, to abort tasks, and to monitor for priority change and abort events.

Concepts and Usage

The API is available in both window and worker threads using the scheduler property on the global object. This property can be tested to feature-check for API support.

Overview

The main API method is Scheduler.postTask(), which takes a callback function ("the task") and returns a promise that resolves with the return value of the function, or rejects with an error.

The simplest form of the API is as shown below. This creates a task with default priority user-visible that has a fixed priority and cannot be aborted.

const promise = scheduler.postTask(myTask);

Because the method returns a promise you can wait on its resolution asynchronously using then, and catch errors thrown by the task callback function (or when the task is aborted) using catch. The callback function can be any kind of function (below we demonstrate an arrow function).

scheduler
  .postTask(() => 'Task executing')
  // Promise resolved: log task result when promise resolves
  .then((taskResult) => console.log(`${taskResult}`))
  // Promise rejected: log AbortError or errors thrown by task
  .catch((error) => console.error(`Error: ${error}`));

The same task might be waited on using await/async as shown below (note, this is run in an Immediately Invoked Function Expression (IIFE)):

(async () => {
  try {
    const result = await scheduler.postTask(() => 'Task executing');
    console.log(result);
  } catch (error) {
    // Log AbortError or error thrown in task function
    console.error(`Error: ${error}`);
  }
})();

You can also specify an options object to the postTask() method if you want to change the default behavior. The options are:

  • priority This allows you to specify a particular immutable priority. Once set, the priority cannot be changed.
  • signal This allows you to specify a signal, which may be either a TaskSignal or AbortSignal The signal is associated with a controller, which can be used to abort the task. A TaskSignal can also be used to set and change the task priority if the task is mutable.
  • delay This allows you to specify the delay before the task is added for scheduling, in milliseconds.

The same example as above with a priority option would look like this:

scheduler
  .postTask(() => 'Task executing', { priority: 'user-blocking' })
  .then((taskResult) => console.log(`${taskResult}`)) // Log the task result
  .catch((error) => console.error(`Error: ${error}`)); // Log any errors

Task priorities

Scheduled tasks are run in priority order, followed by the order that they were added to the scheduler queue.

There are just three priorities, which are listed below (ordered from highest to lowest):

user-blocking

Tasks that stop users from interacting with the page. This includes rendering the page to the point where it can be used, or responding to user input.

user-visible

Tasks that are visible to the user but not necessarily blocking user actions. This might include rendering non-essential parts of the page, such as non-essential images or animations.

This is the default priority.

background

Tasks that are not time-critical. This might include log processing or initializing third party libraries that aren't required for rendering.

Mutable and immutable task priority

There are many use cases where the task priority never needs to change, while for others it does. For example fetching an image might change from a background task to user-visible as a carousel is scrolled into the viewing area.

Task priorities can be set as static (immutable) or dynamic (modifiable) depending on the arguments passed to Scheduler.postTask().=

Task priority is immutable if a value is specified in the options.priority argument. The given value will be used for the task priority and cannot be changed.

The priority is modifiable only if a TaskSignal is passed to the options.signal argument and options.priority is not set. In this case the task will take its initial priority from the signal priority, and the priority can subsequently be changed by calling TaskController.setPriority() on the controller associated with the signal.

If the priority is not set with options.priority or by passing a TaskSignal to options.signal then it defaults to user-visible (and is by definition immutable)

Note that a task that needs to be aborted must set options.signal to either TaskSignal or AbortSignal. However for a task with an immutable priority, AbortSignal more clearly indicates that the task priority cannot be changed using the signal.

Interfaces

Scheduler

Interface with method for adding prioritized tasks to be scheduled. An object of this interface is available on the Window or WorkerGlobalScope global objects (this.scheduler).

TaskController

Interface that supports both aborting a task and changing its priority.

TaskSignal

Interface for a signal object that allows you to abort a task and change its priority, if required, using a TaskController object.

TaskPriorityChangeEvent

The interface for the prioritychange event, sent when the priority for a task is changed.

Note: If the task priority never needs to be changed, you can use an AbortController and its associated AbortSignal instead of TaskController and TaskSignal.

Extensions to other interfaces

The Prioritized Task Scheduling API extends the following APIs, adding the listed features:

scheduler

This property is the entry point for using this API. It is implemented on Window and WorkerGlobalScope, making an instance of Scheduler available through this in most scopes.

Examples

Note that the examples below use mylog() to write to a text area. The code for the log area and method is generally hidden in order to not to distract from more relevant code.

// hidden logger code - simplifies example
let log = document.getElementById('log');
function mylog(text) {
  log.textContent += `${text}\n`;
}

Feature checking

Check whether prioritized task scheduling is supported by testing for the scheduler property in the global "this" exposed to the current scope.

The code below prints "Feature: Supported" if the API is supported on this browser.

// Check that feature is supported
if ('scheduler' in this) {
  mylog('Feature: Supported');
} else {
  mylog('Feature: NOT Supported');
}

Basic usage

Tasks are posted using Scheduler.postTask(), specifying a callback function (task) in the first argument, and an optional second argument that can be used to specify a task priority, signal, and/or delay. The method returns a Promise that resolves with the return value of the callback function, or rejects with either an abort error or an error thrown in the function.

Because it returns a promise, Scheduler.postTask() can be chained with other promises. Below we show how to wait on the promise to resolve using then. This uses the default priority (user-visible).

// A function that defines a task
function myTask() {
  return 'Task 1: user-visible';
}

if ('scheduler' in this) {
  // Post task with default priority: 'user-visible' (no other options)
  // When the task resolves, Promise.then() logs the result.
  scheduler.postTask(myTask).then((taskResult) => mylog(`${taskResult}`));
}

The method can also be used with await inside an async function. The code below shows how you might use this approach to wait on a user-blocking task.

function myTask2() {
  return 'Task 2: user-blocking';
}

async function runTask2() {
  const result = await scheduler.postTask(myTask2, {
    priority: 'user-blocking',
  });
  mylog(result); // Logs 'Task 2: user-blocking'.
}
runTask2();

In some cases you might not need to wait on completion at all. For simplicity many of the examples here simply log the result as the task executes.

// A function that defines a task
function myTask3() {
  mylog('Task 3: user-visible');
}

if ('scheduler' in this) {
  // Post task and log result when it runs
  scheduler.postTask(myTask3);
}

The log below shows the output of the three tasks above. Note that the order they are run depends on the priority first, and then the declaration order.

Permanent priorities

Task priorities may be set using priority parameter in the optional second argument. Priorities that are set in this way are immutable (cannot be changed).

Below we post two groups of three tasks, each member in reverse order of priority. The final task has the default priority. When run, each task simply logs it's expected order (we're not waiting on the result because we don't need to in order to show execution order).

if ('scheduler' in this) {
  // three tasks, in reverse order of priority
  scheduler.postTask(() => mylog('bckg 1'), { priority: 'background' });
  scheduler.postTask(() => mylog('usr-vis 1'), { priority: 'user-visible' });
  scheduler.postTask(() => mylog('usr-blk 1'), { priority: 'user-blocking' });

  // three more tasks, in reverse order of priority
  scheduler.postTask(() => mylog('bckg 2'), { priority: 'background' });
  scheduler.postTask(() => mylog('usr-vis 2'), { priority: 'user-visible' });
  scheduler.postTask(() => mylog('usr-blk 2'), { priority: 'user-blocking' });

  // Task with default priority: user-visible
  scheduler.postTask(() => mylog('usr-vis 3 (default)'));
}

The output below shows that the tasks are executed in priority order, and then declaration order.

Changing task priorities

Task priorities can also take their initial value from a TaskSignal passed to postTask() in the optional second argument. If set in this way, the priority of the task can then be changed using the controller associated with the signal.

Note: Setting and changing task priorities using a signal only works when the options.priority argument to postTask() is not set, and when the options.signal is a TaskSignal (and not an AbortSignal).

The code below first shows how to create a TaskController, setting the initial priority of its signal to user-blocking in the TaskController() constructor.

The code then uses addEventListener() to add an event listener to the controller's signal (we could alternatively use the TaskSignal.onprioritychange property to add an event handler). The event handler uses previousPriority on the event to get the original priority and TaskSignal.priority on the event target to get the new/current priority.

The task is then posted, passing in the signal, and then we immediately change the priority to background by calling TaskController.setPriority() on the controller.

if ('scheduler' in this) {
  // Create a TaskController, setting its signal priority to 'user-blocking'
  const controller = new TaskController({ priority: 'user-blocking' });

  // Listen for 'prioritychange' events on the controller's signal.
  controller.signal.addEventListener('prioritychange', (event) => {
    const previousPriority = event.previousPriority;
    const newPriority = event.target.priority;
    mylog(`Priority changed from ${previousPriority} to ${newPriority}.`);
  });

  // Post task using the controller's signal.
  // The signal priority sets the initial priority of the task
  scheduler.postTask(() => mylog('Task 1'), { signal: controller.signal });

  // Change the priority to 'background' using the controller
  controller.setPriority('background');
}

The output below demonstrates that the priority was successfully changed to background from user-blocking. Note that in this case the priority is changed before the task is executed, but it could equally have been changed while the task was running.

Aborting tasks

Tasks can be aborted using either TaskController and AbortController, in exactly the same way. The only difference is that you must use TaskController if you also want to set the task priority.

The code below creates a controller and passes its signal to the task. The task is then immediately aborted. This causes the promise to be rejected with an AbortError, which is caught in the catch block and logged. Note that we could also have listened for the abort event fired on the TaskSignal or AbortSignal and logged the abort there.

if ('scheduler' in this) {
  // Declare a TaskController with default priority
  const abortTaskController = new TaskController();
  // Post task passing the controller's signal
  scheduler
    .postTask(() => mylog('Task executing'), {
      signal: abortTaskController.signal,
    })
    .then((taskResult) => mylog(`${taskResult}`)) // This won't run!
    .catch((error) => mylog(`Error: ${error}`)); // Log the error

  // Abort the task
  abortTaskController.abort();
}

The log below shows the aborted task.

Delaying tasks

Tasks can be delayed by specifying an integer number of milliseconds in the options.delay parameter to postTask(). This effectively adds the task to the prioritized queue on a timeout, as might be created using setTimeout(). The delay is the minimum amount of time before the task is added to the scheduler; it may be longer.

The code below shows two tasks added (as arrow functions) with a delay.

if ('scheduler' in this) {
  // Post task as arrow function with delay of 2 seconds
  scheduler.
    postTask(() => 'Task delayed by 2000ms', { delay: 2000 })
    .then((taskResult) => mylog(`${taskResult}`));
  scheduler
    .postTask(() => 'Next task should complete in about 2000ms', { delay: 1 })
    .then((taskResult) => mylog(`${taskResult}`));
}

Refresh the page. Note that the second string appears in log after about 2 seconds.

Specifications

Specification
Prioritized Task Scheduling
# scheduler

Browser compatibility

BCD tables only load in the browser

See also