The setHeaderValue() method of the NavigationPreloadManager interface sets the value of the Service-Worker-Navigation-Preload header that will be sent with requests resulting from a fetch() operation made during service worker navigation preloading. It returns an empty Promise that resolves with undefined.

The presence of the Service-Worker-Navigation-Preload header in preloading requests allows servers to configure the returned resource differently for preloading fetch requests than from normal fetch requests. The default directive is set to true: this method allows the possibility of configuring multiple different responses to preload requests.

value

An arbitrary string value, which the target server uses to determine what should returned for the requested resource.

A Promise that resolves with undefined.

InvalidStateError DOMException

There is no active worker associated with the registration to which this NavigationPreloadManager belongs.

The code below demonstrates how the value might be set.

navigator.serviceWorker.ready
  .then((registration) =>
    registration.navigationPreload.setHeaderValue(newValue),
  )
  .then(() => console.log("Done!"))
  .catch((e) =>
    console.error(`NavigationPreloadManager not supported: ${e.message}`),
  );