The getState() method of the NavigationPreloadManager interface returns a Promise that resolves to an object with properties that indicate whether preload is enabled and what value will be sent in the Service-Worker-Navigation-Preload HTTP header.

None.

A Promise that resolves with an object that has the following properties:

enabled

true if preloading is enabled, and false otherwise.

headerValue

A string containing the value that will be sent in the Service-Worker-Navigation-Preload HTTP header following a preloading fetch(). This defaults to true unless the value was changed using NavigationPreloadManager.setHeaderValue().

InvalidStateError DOMException

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

The code below shows a request for the current state, made once the service worker is ready.

navigator.serviceWorker.ready
  .then((registration) => registration.navigationPreload.getState())
  .then((state) => {
    console.log(state.enabled); // boolean
    console.log(state.headerValue); // string
  })
  .catch((e) =>
    console.error(`NavigationPreloadManager not supported: ${e.message}`),
  );