Test your knowledge: The Beacon API Landscape

Question 1 of 6

Chrome 80 (Feb 2020) blocked something during page dismissal. What exactly?

The old analytics pattern relied on a request that froze navigation until it finished.

Answer options A Asynchronous XMLHttpRequest on unload B Synchronous XMLHttpRequest during page dismissal C All network requests from background tabs D navigator.sendBeacon without CORS headers

Synchronous XHR blocked the main thread while the page was being torn down — the browser could not navigate away until the request completed. Chrome 80 killed the pattern; the Beacon API is the spec's replacement.

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Question 2 of 6

Where is the canonical, current Beacon API specification text?

The series' editorial rule: spec sources only — and the spec lives on GitHub.

Answer options A MDN's sendBeacon reference page B The W3C Editor's Draft at w3c.github.io/beacon C The WHATWG Fetch Standard's Beacon chapter D The Chrome Developers analytics guide

The W3C Beacon spec is a Candidate Recommendation Draft (Aug 2022) with an Editor's Draft updated 11 March 2026 at https://w3c.github.io/beacon/. MDN is documentation, not the spec; Fetch defines the machinery the Beacon spec builds on.

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Question 3 of 6

Under the hood, `navigator.sendBeacon()` is mandated to construct which request?

The spec says the user agent SHOULD use the Fetch infrastructure — with one special flag and one special initiator.

Answer options A A fetch() with keepalive: true and initiator type "beacon" B A synchronous XMLHttpRequest with a keepalive header C A WebSocket message flushed on unload D A fetch() with cache: 'no-store' and initiator "navigation"

The Beacon spec's processing model constructs a Fetch-standard Request with method POST, keepalive: true, initiator type "beacon", credentials mode "include", and an auto-derived CORS mode. Beacons are fetches with a badge.

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Question 4 of 6

Since when has `navigator.sendBeacon` been universally available across browsers?

The 2024 date belongs to a different feature in the same family.

Answer options A 2014 — Chrome 39 was the last holdout B 2017 — Chrome 39 / Firefox 31 / Safari 11.1 covered everyone C 2020 — Chrome 80 forced every browser to ship it D 2024 — Firefox 133 completed the set

Chrome 39 (2014), Firefox 31 (2014), and Safari 11.1 (2017) ship sendBeacon — universal since 2017. Firefox 133 (Nov 2024) is notable for shipping fetch(keepalive), the superset, not sendBeacon itself.

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Question 5 of 6

What does the universal feature-detection recipe check beyond `typeof navigator.sendBeacon === 'function'`?

Choosing a transport means knowing all the rungs of the fallback ladder.

Answer options A Only the user-agent string, cached in localStorage B fetch(keepalive) availability, lifecycle event support, and fetchLater presence C The server's response to a probe beacon D Nothing — sendBeacon support implies everything else

The recipe probes sendBeacon, fetch(keepalive) (Firefox lacked it until v133), visibilitychange/pagehide/pageshow, unload presence, worker context, service worker fetch interception, and fetchLater — the full capability matrix for choosing a transport.

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Question 6 of 6

What is the status of the `unload` event in Chrome 154 (Sep 2026)?

The rollout started in Chrome 135 and finished in 2026.

Answer options A Fully deprecated — 100% rollout, it no longer fires B Still fires, but logs a deprecation warning C Replaced by beforeunload for analytics D Only disabled for cross-origin iframes

Chrome began deprecating unload in 135 (Mar 2025) and completed the 100% rollout in Chrome 154 (Sep 22, 2026). Analytics must move to the visibilitychange → hidden transition; cleanup belongs in pagehide.

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Test your knowledge: The Beacon API Landscape