ThetransferSize 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 byRFC7230).

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

Value

ThetransferSize 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 theresponse payload body (RFC7230).
• 0 if the resource was instantaneously retrieved from a cache.
• 0 if the resource is a cross-origin request and noTiming-Allow-Origin HTTP response header is used.

Examples

Checking if a cache was hit

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

Example using aPerformanceObserver, which notifies of newresource performance entries as they are recorded in the browser's performance timeline. Use thebuffered 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 usingPerformance.getEntriesByType(), which only showsresource 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 thetransferSize property is0 and wasn't loaded from a local cache, the resource might be a cross-origin request. To expose cross-origin content size information, theTiming-Allow-Origin HTTP response header needs to be set.

For example, to allowhttps://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