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Long Tasks API

Motivation

The experimental Long Tasks API gives us visibility into tasks that take 50 milliseconds or more. The 50 ms threshold comes from the RAIL Model, in particular the "Response: process events in under 50 ms" section.

Tasks that block the main thread for 50 ms or more cause, among other issues:

  • Delayed "Time to interactive".
  • High/variable input latency.
  • High/variable event handling latency.
  • Janky animations and scrolling.

Concepts

Some key terms or ideas that are utilized by the Long Tasks API.

Long task

Any uninterrupted period where the main UI thread is busy for 50 ms or longer. Common examples include:

  • Long running event handlers.
  • Expensive reflows and other re-renders.
  • Work the browser does between different turns of the event loop that exceeds 50 ms.

Culprit browsing context container

The "culprit browsing context container", or "the container" for short, is the top level page, iframe, embed or object that the task occurred within.

Attributions

A list of containers that the task occurred within. For tasks that don't occur within the top level page, the containerId, containerName and containerSrc fields may provide information as to the source of the task.

Usage

var observer = new PerformanceObserver(function(list) {
    var perfEntries = list.getEntries();
    for (var i = 0; i < perfEntries.length; i++) {
        // Process long task notifications:
        // report back for analytics and monitoring
        // ...
    }
});
// register observer for long task notifications
observer.observe({entryTypes: ["longtask"]});
// Long script execution after this will result in queueing
// and receiving "longtask" entries in the observer.

Interfaces

PerformanceLongTaskTiming

Reports instances of long tasks.

TaskAttributionTiming

Returns information about the work involved in a long task and its associate frame context.

Specifications

Specification Status Comment
Long Tasks API 1 Working Draft Initial definition.

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
Long_Tasks_API
58
79
No
No
45
No
58
58
No
43
No
7.0
containerId
58
79
No
No
45
No
58
58
No
43
No
7.0
containerName
58
79
No
No
45
No
58
58
No
43
No
7.0
containerSrc
58
79
No
No
45
No
58
58
No
43
No
7.0
containerType
58
79
No
No
45
No
58
58
No
43
No
7.0
toJSON
58
79
No
No
45
No
58
58
No
43
No
7.0
Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
Long_Tasks_API
58
79
No
No
45
No
58
58
No
43
No
7.0
attribution
58
79
No
No
45
No
58
58
No
43
No
7.0
toJSON
58
79
No
No
45
No
58
58
No
43
No
7.0

PerformanceLongTaskTiming

BCD tables only load in the browser

TaskAttributionTiming

BCD tables only load in the browser

See also

© 2005–2021 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/Long_Tasks_API