PerformanceLongTaskTiming
Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The PerformanceLongTaskTiming
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:
- Delayed Time 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, the TaskAttributionTiming
interface provides the containerId
, containerName
and containerSrc
properties, which may provide more information about the source of the task.
Inheritance
PerformanceLongTaskTiming
inherits from PerformanceEntry
.
Instance properties
This interface extends the following PerformanceEntry
properties for long task timing performance entry types by qualifying them as follows:
PerformanceEntry.duration
Read only Experimental-
Returns a
DOMHighResTimeStamp
representing the elapsed time between the start and end of the task, with a 1ms granularity. PerformanceEntry.entryType
Read only Experimental-
Always returns
"longtask"
PerformanceEntry.name
Read only Experimental-
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.startTime
Read only Experimental-
Returns a
DOMHighResTimeStamp
representing the time when the task started.
This interface also supports the following properties:
PerformanceLongTaskTiming.attribution
Read only Experimental-
Returns a sequence of
TaskAttributionTiming
instances.
Instance methods
PerformanceLongTaskTiming.toJSON()
Experimental-
Returns a JSON representation of the
PerformanceLongTaskTiming
object.
Examples
Getting long tasks
To get long task timing information, create a PerformanceObserver
instance and then call its observe()
method, passing in "longtask"
as the value of the type
option. You also need to set buffered
to true
to get access to long tasks the user agent buffered while constructing the document. The PerformanceObserver
object's callback will then be called with a list of PerformanceLongTaskTiming
objects which you can analyze.
js
const observer = new PerformanceObserver((list) => {
list.getEntries().forEach((entry) => {
console.log(entry);
});
});
observer.observe({ type: "longtask", buffered: true });
Specifications
Specification |
---|
Long Tasks API # sec-PerformanceLongTaskTiming |
Browser compatibility
BCD tables only load in the browser