LayoutShift

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.

TheLayoutShift interface of thePerformance API provides insights into the layout stability of web pages based on movements of the elements on the page.

Description

A layout shift happens when any element that is visible in the viewport changes its position between two frames. These elements are described as beingunstable, indicating a lack of visual stability.

The Layout Instability API provides a way to measure and report on these layout shifts. All tools for debugging layout shifts, including those in the browser's developer tools, use this API. The API can also be used to observe and debug layout shifts by logging the information to the console, to send the data to a server endpoint, or to web page analytics.

Performance tools can use this API to calculate aCLS score.

Instance properties

This interface extends the followingPerformanceEntry properties by qualifying them as follows:

PerformanceEntry.durationRead onlyExperimental: Always returns0 (the concept of duration does not apply to layout shifts).
PerformanceEntry.entryTypeRead onlyExperimental: Always returns"layout-shift".
PerformanceEntry.nameRead onlyExperimental: Always returns"layout-shift".
PerformanceEntry.startTimeRead onlyExperimental: Returns a DOMHighResTimeStamp representing the time when the layout shift started.

This interface also supports the following properties:

LayoutShift.valueExperimental: Returns the layout shift score calculated as the impact fraction (fraction of the viewport that was shifted) multiplied by the distance fraction (distance moved as a fraction of viewport).
LayoutShift.hadRecentInputExperimental: Returnstrue if lastInputTime is less than 500 milliseconds in the past.
LayoutShift.lastInputTimeExperimental: Returns the time of the most recent excluding input (user input that would exclude this entry as a contributor to the CLS score) or0 if no excluding input has occurred.
LayoutShift.sourcesExperimental: Returns an array of LayoutShiftAttribution objects with information on the elements that were shifted.

Instance methods

LayoutShift.toJSON()Experimental: Converts the properties to JSON.

Examples

Logging layout shift values

The following example shows how to capture layout shifts and log them to the console.

const observer = new PerformanceObserver((list) => {  for (const entry of list.getEntries()) {    // Count layout shifts without recent user input only    if (!entry.hadRecentInput) {      console.log("LayoutShift value:", entry.value);      if (entry.sources) {        for (const { node, currentRect, previousRect } of entry.sources)          console.log("LayoutShift source:", node, {            currentRect,            previousRect,          });      }    }  }});observer.observe({ type: "layout-shift", buffered: true });