PerformanceResourceTiming: transferSize property
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.
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 noTiming-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.
js
const observer = new PerformanceObserver((list) => {
list.getEntries().forEach((entry) => {
if (entry.transferSize === 0 && entry.decodedBodySize > 0) {
console.log(`${entry.name} was loaded from cache`);
}
});
});
observer.observe({ type: "resource", buffered: true });
Example using Performance.getEntriesByType()
, which only shows resource
performance entries present in the browser's performance timeline at the time you call this method:
js
const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
if (entry.transferSize === 0 && entry.decodedBodySize > 0) {
console.log(`${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:
http
Timing-Allow-Origin: https://developer.mozilla.org
Specifications
Specification |
---|
Resource Timing # dom-performanceresourcetiming-transfersize |
Browser compatibility
BCD tables only load in the browser