PerformanceLongTaskTiming

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental:This is anexperimental technology
Check theBrowser compatibility table carefully before using this in production.

ThePerformanceLongTaskTiming interface provides information about tasks that occupy the UI thread for 50 milliseconds or more.

Description

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

DelayedTime to interactive (TTI).
High/variable input latency.
High/variable event handling latency.
Janky animations and scrolling.

A long task is any uninterrupted period where the main UI thread is busy for 50ms 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.

Long tasks refer to "culprit browsing context container", or "the container" for short, which is the top-level page,<iframe>,<embed> or<object> that the task occurred within.

For tasks that don't occur within the top-level page and for figuring out which container is responsible for the long task, theTaskAttributionTiming interface provides thecontainerId,containerName andcontainerSrc properties, which may provide more information about the source of the task.

PerformanceLongTaskTiming inherits fromPerformanceEntry.

Instance properties

This interface extends the followingPerformanceEntry properties for long task timing performance entry types by qualifying them as follows:

PerformanceEntry.durationRead onlyExperimental

Returns a DOMHighResTimeStamp representing the elapsed time between the start and end of the task, with a 1ms granularity.

PerformanceEntry.entryTypeRead onlyExperimental

Always returns"longtask"

PerformanceEntry.nameRead onlyExperimental

Returns one of the following strings referring to the browsing context or frame that can be attributed to the long task:

"cross-origin-ancestor"
"cross-origin-descendant"
"cross-origin-unreachable"
"multiple-contexts"
"same-origin-ancestor"
"same-origin-descendant"
"same-origin"
"self"
"unknown"

PerformanceEntry.startTimeRead onlyExperimental

Returns a DOMHighResTimeStamp representing the time when the task started.

This interface also supports the following properties:

PerformanceLongTaskTiming.attributionRead onlyExperimental: Returns a sequence of TaskAttributionTiming instances.

Instance methods

PerformanceLongTaskTiming.toJSON()Experimental: Returns a JSON representation of thePerformanceLongTaskTiming object.

Examples

Getting long tasks

To get long task timing information, create aPerformanceObserver instance and then call itsobserve() method, passing in"longtask" as the value of thetype option. You also need to setbuffered totrue to get access to long tasks the user agent buffered while constructing the document. ThePerformanceObserver object's callback will then be called with a list ofPerformanceLongTaskTiming objects which you can analyze.

const observer = new PerformanceObserver((list) => {  list.getEntries().forEach((entry) => {    console.log(entry);  });});observer.observe({ type: "longtask", buffered: true });