PerformanceResourceTiming: transferSize property

Value

The transferSize property can have the following values:

• A number representing the size (in octets) of the fetched resource. The size includes the response header fields plus the response payload body (RFC7230).
• 0 if the resource was instantaneously retrieved from a cache.
• 0 if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

Examples

Checking if a cache was hit

For environments not supporting the responseStatus property, the transferSize property can be used to determine cache hits. If transferSize is zero and the resource has a non-zero decoded body size (meaning the resource is same-origin or has Timing-Allow-Origin), the resource was fetched from a local cache.

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) => {
    if (entry.transferSize === 0 && entry.decodedBodySize > 0) {
      console.log(`${entry.name} was loaded from cache`);
    }
  });
});

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) => {
  if (entry.transferSize === 0 && entry.decodedBodySize > 0) {
    console.log(`${entry.name} was loaded from cache`);
  }
});

Cross-origin content size information

If the value of the transferSize property is 0 and wasn't loaded from a local cache, 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

Specifications

Browser compatibility

See also

• Timing-Allow-Origin