Browser support

isorouter is built directly on the browser Navigation API. There is no History API fallback — that's a deliberate design choice, not an omission.

Baseline status

The Navigation API reached Baseline Newly available in early 2026 (Chrome, Edge, Firefox, Safari). It is not yet Widely available, so browsers still in production from before then lack it.

Supported contract

The supported contract is the pure Navigation API. If a browser has it, isorouter works. If it doesn't, load a polyfill (below) — but the polyfill is best-effort, with known gaps. isorouter does not ship or maintain its own History-based shim.

Polyfill (opt-in)

If you must support pre-2026 engines, load a polyfill before starting the router (the "unix way": one small dependency, opt-in):

if (!window.navigation) {
  const { applyPolyfill } = await import(
    "@virtualstate/navigation/apply-polyfill"
  );
  applyPolyfill();
}
router.start(); // or mount <Router> / <RouterView>

@virtualstate/navigation is the only polyfill we're aware of that implements navigate + event.intercept(), which isorouter's interception model depends on. The e2e suite runs the full test matrix a second time with the native Navigation API hidden, against this polyfill, to verify behavioural parity.

Known limitation

As of @virtualstate/navigation@1.0.1-alpha.212, interceptWindowClicks reports downloadRequest: "" (instead of null) for plain <a> clicks, so the navigate event isn't intercepted and the polyfill falls back to a full-page navigation for link clicks. The route still renders correctly (the fresh page load re-runs router.start()), but the transition isn't client-side.

Imperative navigation — router.navigate, back, forward, guards, redirects, lazy loading — is unaffected and works identically to native.

What happens with no Navigation API and no polyfill

• router.start() is a no-op.
• router.navigate(...) throws.
• router.back() / router.forward() are no-ops.

So a feature-detect like the snippet above is all you need to degrade safely.