PerformanceResourceTiming: decodedBodySize property

The decodedBodySize property can have the following values:

• A number representing the size (in octets) received from the fetch (HTTP or cache) of the message body, after removing any applied content encoding.
• 0 if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

If the decodedBodySize and encodedBodySize properties are non-null and differ, the content was compressed (for example, gzip or Brotli).

Example using a PerformanceObserver, which notifies of new resource performance entries as they are recorded in the browser's performance timeline. Use the buffered option to access entries from before the observer creation.

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    const uncompressed =
      entry.decodedBodySize && entry.decodedBodySize === entry.encodedBodySize;
    if (uncompressed) {
      console.log(`${entry.name} was not compressed!`);
    }
  });
});

observer.observe({ type: "resource", buffered: true });

Example using Performance.getEntriesByType(), which only shows resource performance entries present in the browser's performance timeline at the time you call this method:

const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
  const uncompressed =
    entry.decodedBodySize && entry.decodedBodySize === entry.encodedBodySize;
  if (uncompressed) {
    console.log(`${entry.name} was not compressed!`);
  }
});

If the value of the decodedBodySize property is 0, the resource might be a cross-origin request. To expose cross-origin content size information, the Timing-Allow-Origin HTTP response header needs to be set.

For example, to allow https://developer.mozilla.org to see content sizes, the cross-origin resource should send:

Timing-Allow-Origin: https://developer.mozilla.org

• Timing-Allow-Origin