LargestContentfulPaint
Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The LargestContentfulPaint
interface provides timing information about the largest image or text paint before user input on a web page.
Description
The key moment this API provides is the Largest Contentful Paint (LCP) metric. It provides the render time of the largest image or text block visible within the viewport, recorded from when the page first begins to load. The following elements are considered when determining the LCP:
<img>
elements.<image>
elements inside an SVG.- The poster images of
<video>
elements. - Elements with a
background-image
. - Groups of text nodes, such as
<p>
.
To measure render times of other elements, use the PerformanceElementTiming
API.
Additional key paint moments are provided by the PerformancePaintTiming
API:
- First paint (FP): Time when anything is rendered. Note that the marking of the first paint is optional, not all user agents report it.
- First contentful paint (FCP): Time when the first bit of DOM text or image content is rendered.
LargestContentfulPaint
inherits from PerformanceEntry
.
Instance properties
This interface extends the following PerformanceEntry
properties by qualifying and constraining the properties as follows:
PerformanceEntry.entryType
Read only Experimental-
Returns "
largest-contentful-paint
". PerformanceEntry.name
Read only Experimental-
Always returns an empty string.
PerformanceEntry.startTime
Read only Experimental-
Returns the value of this entry's
renderTime
if it is not0
, otherwise the value of this entry'sloadTime
. PerformanceEntry.duration
Read only Experimental-
Returns
0
, asduration
is not applicable to this interface.
It also supports the following properties:
LargestContentfulPaint.element
Read only Experimental-
The element that is the current largest contentful paint.
LargestContentfulPaint.renderTime
Read only Experimental-
The time the element was rendered to the screen. May not be available if the element is a cross-origin image loaded without the
Timing-Allow-Origin
header. LargestContentfulPaint.loadTime
Read only Experimental-
The time the element was loaded.
LargestContentfulPaint.size
Read only Experimental-
The intrinsic size of the element returned as the area (width * height).
LargestContentfulPaint.id
Read only Experimental-
The id of the element. This property returns an empty string when there is no id.
LargestContentfulPaint.url
Read only Experimental-
If the element is an image, the request url of the image.
Instance methods
This interface also inherits methods from PerformanceEntry
.
LargestContentfulPaint.toJSON()
Experimental-
Returns a JSON representation of the
LargestContentfulPaint
object.
Examples
Observing the largest contentful paint
In the following example, an observer is registered to get the largest contentful paint while the page is loading. The buffered
flag is used to access data from before observer creation.
The LCP API analyzes all content it finds (including content that is removed from the DOM). When new largest content is found, it creates a new entry. It stops searching for larger content when scroll or input events occur, since these events likely introduce new content on the website. Thus the LCP is the last performance entry reported by the observer.
js
const observer = new PerformanceObserver((list) => {
const entries = list.getEntries();
const lastEntry = entries[entries.length - 1]; // Use the latest LCP candidate
console.log("LCP:", lastEntry.startTime);
console.log(lastEntry);
});
observer.observe({ type: "largest-contentful-paint", buffered: true });
Cross-origin image render time
For security reasons, the value of the renderTime
property is 0
if the resource is a cross-origin request. Instead the loadTime
is exposed. To expose cross-origin render time information, the Timing-Allow-Origin
HTTP response header needs to be set.
For example, to allow https://developer.mozilla.org
to see renderTime
, the cross-origin resource should send:
http
Timing-Allow-Origin: https://developer.mozilla.org
Like in the code example, you can use startTime
, which returns the value of the entry's renderTime
if it is not 0
, and otherwise the value of this entry's loadTime
. However, it is recommended to set the Timing-Allow-Origin
header so that the metrics will be more accurate.
If you use startTime
, you can flag any inaccuracies by checking if renderTime
was used:
js
const isAccurateLCP = entry.renderTime ? true : false;
Specifications
Specification |
---|
Largest Contentful Paint # sec-largest-contentful-paint-interface |
Browser compatibility
BCD tables only load in the browser