The preloadResponse read-only property of the FetchEvent interface returns a Promise that resolves to the navigation preload Response if navigation preload was triggered or undefined otherwise.


var expectedResponse = fetchEvent.preloadResponse;


A Promise that resolves to a Response or otherwise to undefined.


This code snippet is from the navigation preload page. The ServiceWorkerGlobalScope.onfetch event handler listens for the fetch event. When fired, pass a promise that back to the controlled page to FetchEvent.respondWith(). This promise resolves to the first matching URL request in the Cache object. If no match is found, the code checks for a preloaded response. Else it fetches a response from the network.

addEventListener('fetch', event => {
  event.respondWith(async function() {
    // Respond from the cache if we can
    const cachedResponse = await caches.match(event.request);
    if (cachedResponse) return cachedResponse;

    // Else, use the preloaded response, if it's there
    const response = await event.preloadResponse;
    if (response) return response;

    // Else try the network.
    return fetch(event.request);


Specification Status Comment
Service Workers
The definition of 'preloadResponse' in that specification.
Working Draft Initial definition.

Browser compatibility

ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
Chrome Full support 59Edge Full support 18Firefox ? IE No support NoOpera Full support 46Safari No support NoWebView Android Full support 59Chrome Android Full support 59Firefox Android ? Opera Android Full support 43Safari iOS No support NoSamsung Internet Android Full support 7.0


Full support
Full support
No support
No support
Compatibility unknown
Compatibility unknown
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.

See also