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LargestContentfulPaint - Web APIs 编辑

Experimental

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The LargestContentfulPaint interface of the Largest Contentful Paint API provides details about the largest image or text paint before user input on a web page. The timing of this paint is a good heuristic for when the main page content is available during load.

Properties

LargestContentfulPaint.element: The element that is the current largest contentful paint.
LargestContentfulPaint.renderTime: 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: The time the element was loaded.
LargestContentfulPaint.size: The intrinsic size of the element returned as the area (width * height).
LargestContentfulPaint.id: The id of the element. This property returns an empty string when there is no id.
LargestContentfulPaint.url: If the element is an image, the request url of the image.

Methods

LargestContentfulPaint.toJSON(): Returns the above properties as JSON.

Examples

The following example shows how to create a PerformanceObserver that listens for largest-contentful-paint entries and logs the LCP value to the console.

This example also demonstrates how to include buffered entries (those that ocurred before observer() was called), which is done by setting the buffered option to true.

Note that in this example data is only sent to the server when the user leaves the tab.

// Catch errors since some browsers throw when using the new `type` option.
// https://bugs.webkit.org/show_bug.cgi?id=209216
try {
  let lcp;

  const po = new PerformanceObserver((entryList) => {
    const entries = entryList.getEntries();
    const lastEntry = entries[entries.length - 1];

    // Update `lcp` to the latest value, using `renderTime` if it's available,
    // otherwise using `loadTime`. (Note: `renderTime` may not be available on
    // image elements loaded cross-origin without the `Timing-Allow-Origin` header.)
    lcp = lastEntry.renderTime || lastEntry.loadTime;
  });

  po.observe({type: 'largest-contentful-paint', buffered: true});

  // Send data to the server.
  addEventListener('visibilitychange', function fn() {
    if (lcp && document.visibilityState === 'hidden') {
      console.log('LCP:', lcp);
      removeEventListener('visibilitychange', fn, true);
    }
  }, true);
} catch (e) {
  // Do nothing if the browser doesn't support this API.
}