LayoutShift

A layout shift happens when any element that is visible in the viewport changes its position between two frames. These elements are described as being unstable, 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.

Popular performance tools, use this API to calculate a Cumulative Layout Shift (CLS) score.

This interface extends the following PerformanceEntry properties by qualifying them as follows:

PerformanceEntry.duration Read only Experimental

Always returns 0 (the concept of duration does not apply to layout shifts).

PerformanceEntry.entryType Read only Experimental

Always returns "layout-shift".

PerformanceEntry.name Read only Experimental

Always returns "layout-shift".

PerformanceEntry.startTime Read only Experimental

Returns a DOMHighResTimeStamp representing the time when the layout shift started.

This interface also supports the following properties:

LayoutShift.value Experimental

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.hadRecentInput Experimental

Returns true if lastInputTime is less than 500 milliseconds in the past.

LayoutShift.lastInputTime Experimental

Returns the time of the most recent excluding input (user input that would exclude this entry as a contributor to the CLS score) or 0 if no excluding input has occurred.

LayoutShift.sources Experimental

Returns an array of LayoutShiftAttribution objects with information on the elements that were shifted.

LayoutShift.toJSON() Experimental

Converts the properties to JSON.

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 });

• LayoutShiftAttribution
• Cumulative Layout Shift (CLS)