Test your knowledge: The Core API

Question 1 of 6

Which HTTP method does `sendBeacon` use, and can you change it?

The spec hard-codes it — the API has no method field at all.

Answer options A POST — there is no method parameter B POST, overridable via the third options argument C GET when the body is null D PUT, matching the fire-and-forget contract

sendBeacon always POSTs. The spec wires it to fetch() with method 'POST'. For PUT/PATCH/DELETE you need fetch(keepalive) — the fetch(keepalive) superset.

Review this lesson

Question 2 of 6

`navigator.sendBeacon()` returns `true`. What does that guarantee?

The spec asks: 'Was the request successfully queued into the fetch group's keepalive request list?'

Answer options A The server received the payload B The request was successfully queued into the fetch group's keepalive list C The request will survive any future quota pressure D The response was a 2xx status

The boolean reflects QUEUEING success only. Delivery is fire-and-forget: no response callback, no status, no retry. A beacon that later fails at the network layer simply vanishes.

Review this lesson

Question 3 of 6

A cross-origin beacon fails its CORS preflight. What does the caller observe?

The return already happened. There is nowhere to deliver an error later.

Answer options A The promise rejects with a TypeError B sendBeacon returns false at call time C Nothing — the boolean was already true; the drop is silent D A beaconerror event fires on navigator

sendBeacon returns a synchronous boolean about queueing. A later CORS preflight failure has no channel to surface through — no promise, no event, no callback. The request is silently dropped while the caller believes it was sent.

Review this lesson

Question 4 of 6

What happens to the `url` argument of `sendBeacon`?

You can pass a URL object, a relative path, or a string — and one part of the URL never survives.

Answer options A It is coerced to USVString and resolved against the document base URL; fragments are stripped B It must be absolute, or the call throws C Query strings are stripped along with fragments D It is encoded as a Blob URL before transmission

The first argument is coerced to USVString and resolved against document.baseURI. Fragments (#section) are stripped; query strings are preserved. An empty url throws TypeError.

Review this lesson

Question 5 of 6

Where can you call `navigator.sendBeacon`?

Check the IDL: partial interface Navigator { boolean sendBeacon(…) } — and WorkerNavigator is NOT listed.

Answer options A Any JS realm — Window, DedicatedWorker, SharedWorker, ServiceWorker B Window contexts only — the mixin is included by Navigator, not WorkerNavigator C Window and DedicatedWorker, but not ServiceWorker D Only secure contexts over HTTPS

The Beacon spec defines sendBeacon on a partial Navigator interface. WorkerNavigator is not extended by it, so no worker context (dedicated, shared, or service) can originate a beacon. Workers must use fetch(keepalive).

Review this lesson

Question 6 of 6

What is the design goal of the isomorphic sender recipe ( `isomorphic-sender.js`)?

Window has sendBeacon; workers have fetch(keepalive). The recipe bridges the two.

Answer options A One API that works identically in Window and Worker contexts B A polyfill that adds sendBeacon to Internet Explorer C A server-side Node implementation of the Beacon protocol D A proxy that mirrors beacons to two collectors

The isomorphic sender probes the realm: Window uses navigator.sendBeacon, Worker contexts fall back to fetch(url, {method:'POST', body, keepalive:true}) — one boolean-returning sendBeacon-style API everywhere.

Review this lesson

Test your knowledge: The Core API