Inside a `navigate` event listener on `window.navigation`, when is `event.canIntercept` false, and what should your code do in that case?

## What the flag means `event.canIntercept` on a `NavigateEvent` answers exactly one question: may this navigation be converted into a same-document navigation that your JavaScript completes? The browser computes it before your listener runs, and it is not advisory — `intercept()` throws a `DOMException` when the flag is `false`. ## The cases that make it false **Cross-origin destination.** A link to another site cannot be rendered inside your document; the same-origin policy is the whole reason. `event.destination.url` will have a different origin from the current document. This is the case that bites unguarded routers: every outbound link on the page starts throwing. **Download requests.** When the navigation came from a link with a `download` attribute, `event.downloadRequest` is a string rather than `null` — the requested filename, or an empty string if the attribute had no value. Downloads are not page navigations, so there is nothing to intercept. The flag is a genuinely useful hook even so: it is the one reliable place to notice "the user started a download" without wiring a click handler onto every link. **Cross-document traversals.** A back or forward navigation to an entry that was itself a full document load cannot be replayed as a same-document render — the browser has to restore or refetch that document. `event.destination.sameDocument` is `false` in that case. **A document that is no longer fully active**, for example one sitting inside a detached or hidden-away navigable. Note what is *not* in the list. A same-origin navigation to a completely different path — the ordinary SPA case — is interceptable even though the browser would otherwise fetch a new document. A same-origin form submission is interceptable too, and `event.formData` gives you the submitted entries. Those are the navigations the API exists to hand you. ## The shape of a correct listener ```js navigation.addEventListener("navigate", (event) => { if (event.downloadRequest !== null) return; // browser downloads the file if (!event.canIntercept) return; // cross-origin, or a real document load event.intercept({ handler: () => renderRoute(event.destination.url) }); }); ``` Guard first, intercept second. Returning early is not a failure mode — it is the browser doing the navigation the way it always did, which is exactly what you want for a link to another site. A subtlety worth stating: `canIntercept` is about *taking over*, not about *observing* or *cancelling*. The `navigate` event still fires for a cross-origin link click, so you can record the outbound click. And when `event.cancelable` is `true` you may still call `preventDefault()` to stop the navigation, which is how a form guard prevents the user leaving with unsaved edits regardless of where they were heading. ## Fragment-only navigations `event.hashChange` is `true` when only the fragment differs. These are same-document and interceptable, but you usually do not want to: intercepting means you take responsibility for scrolling to the target element. Many routers add `if (event.hashChange) return;` as a third guard so the browser keeps handling in-page anchors natively. ## Why interviewers ask It separates people who have read about the API from people who have shipped it. The demo snippets in blog posts often omit the guards; the first bug report from production is always "clicking a link to our docs site does nothing and the console shows an exception". Being able to name the four cases — cross-origin, download, cross-document traversal, inactive document — shows you have hit that and understand why the platform draws the line where it does.