PerformanceElementTiming

Description

The aim of the Element Timing API is to give web developers or analytics tools the ability to measure rendering timestamps of critical elements on a page.

The API supports timing information on the following elements:

• <img> elements,
• <image> elements inside an <svg>,
• poster images of <video> elements,
• elements which have a contentful background-image property with a URL value for a resource that is actually available, and
• groups of text nodes, such as a <p>.

The author flags an element for observation by adding the elementtiming attribute on the element.

PerformanceElementTiming inherits from PerformanceEntry.

Instance properties

This interface directly defines the following properties:

PerformanceElementTiming.element Read only

An Element representing the element we are returning information about.

PerformanceElementTiming.id Read only

A string which is the id of the element.

PerformanceElementTiming.identifier Read only

A string which is the value of the elementtiming attribute on the element.

PerformanceElementTiming.intersectionRect Read only

A DOMRectReadOnly which is the rectangle of the element within the viewport.

PerformanceElementTiming.loadTime Read only

A DOMHighResTimeStamp with the loadTime of the element.

PerformanceElementTiming.naturalHeight Read only

An unsigned 32-bit integer (unsigned long) which is the intrinsic height of the image if this is applied to an image, 0 for text.

PerformanceElementTiming.naturalWidth Read only

An unsigned 32-bit integer (unsigned long) which is the intrinsic width of the image if this is applied to an image, 0 for text.

PerformanceElementTiming.paintTime

Returns the timestamp when the rendering phase ended and the paint phase started.

PerformanceElementTiming.presentationTime

Returns the timestamp when the element was actually drawn on the screen.

PerformanceElementTiming.renderTime Read only

A DOMHighResTimeStamp with the renderTime of the element.

PerformanceElementTiming.url Read only

A string which is the initial URL of the resources request for images, 0 for text.

It also extends the following PerformanceEntry properties, qualifying and constraining them as described:

PerformanceEntry.duration Read only

Always returns 0 as duration does not apply to this interface.

PerformanceEntry.entryType Read only

Always returns "element".

PerformanceEntry.name Read only

Returns "image-paint" for images and "text-paint" for text.

PerformanceEntry.startTime Read only

Returns the value of this entry's renderTime if it is not 0, otherwise the value of this entry's loadTime.

Instance methods

PerformanceElementTiming.toJSON()

Returns a JSON representation of the PerformanceElementTiming object.

Examples

Observing render time of specific elements

In this example two elements are being observed by adding the elementtiming attribute. A PerformanceObserver is registered to get all performance entries of type "element" and the buffered flag is used to access data from before observer creation.

<img src="image.jpg" elementtiming="big-image" />
<p elementtiming="text" id="text-id">text here</p>

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

Two entries will be output to the console. The first containing details of the image, the second with details of the text node.

Observing separate paint and presentation timings

The paintTime and presentationTime properties enable you to retrieve specific timings for the paint phase starting and the element being drawn on the screen. The paintTime is broadly interoperable, whereas the presentationTime is implementation-dependent.

This example uses a PerformanceObserver to observe all performance entries of type "element" (remember that, to be observed, elements need to have elementtiming attributes set on them). We check for paintTime and presentationTime support and retrieve those values if they are available. In non-supporting browsers, the code retrieves the renderTime or loadTime, depending on what is supported.

const observer = new PerformanceObserver((list) => {
  const entries = list.getEntries();
  entries.forEach((entry) => {
    if (entry.presentationTime) {
      console.log(
        "Element paintTime:",
        entry.paintTime,
        "Element presentationTime:",
        entry.presentationTime,
      );
    } else if (entry.paintTime) {
      console.log("Element paintTime:", entry.paintTime);
    } else if (entry.renderTime) {
      console.log("Element renderTime:", entry.renderTime);
    } else {
      console.log("Element loadTime", entry.loadTime);
    }
  });
});
observer.observe({ type: "element", buffered: true });

Specifications

Browser compatibility

See also

• elementtiming HTML attribute