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Showing content from http://developer.mozilla.org/en-US/docs/Web/API/PerformanceResourceTiming/transferSize below:

PerformanceResourceTiming: transferSize property - Web APIs

Baseline 2023

Newly available

Note: This feature is available in Web Workers.

The transferSize read-only property represents the size (in octets) of the fetched resource. The size includes the response header fields plus the response payload body (as defined by RFC7230).

If the resource is fetched from a local cache, or if it is a cross-origin resource, this property returns zero.

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.

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`);
  }
});

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

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