leptos-rs/leptos · error

no `finished` property on ViewTransition

Error message

no `finished` property on ViewTransition

What it means

During navigation the router uses the browser's View Transitions API and reads `viewTransition.finished` via `Reflect::get`, expecting a Promise. If the returned object does not expose `finished` (unsupported/partially implemented browser, or a mocked/undefined return from `document.startViewTransition`), the `expect` panics.

Source

Thrown at router/src/lib.rs:192

        if is_back_navigation {
            _ = class_list.add_1("router-back");
        }
        match svt {
            Ok(svt) => {
                let cb = Closure::once_into_js(Box::new(move || {
                    fun();
                }));
                match svt.call1(
                    document.unchecked_ref(),
                    cb.as_ref().unchecked_ref(),
                ) {
                    Ok(view_transition) => {
                        let class_list = document_element.class_list();
                        let finished = Reflect::get(
                            &view_transition,
                            &JsValue::from_str("finished"),
                        )
                        .expect("no `finished` property on ViewTransition")
                        .unchecked_into::<Promise>();
                        let cb = Closure::new(Box::new(move |_| {
                            if is_back_navigation {
                                class_list.remove_1("router-back").unwrap();
                            }
                            class_list
                                .remove_1(&format!("router-outlet-{level}"))
                                .unwrap();
                        })
                            as Box<dyn FnMut(JsValue)>);
                        _ = finished.then(&cb);
                        cb.into_js_value();
                    }
                    Err(e) => {
                        web_sys::console::log_1(&e);
                    }
                }
            }

View on GitHub (pinned to 32d20f6c9d)

Solutions

1. Feature-detect full support and disable view transitions when unavailable: check `document.startViewTransition` exists and that the result has a `finished` property before using it.
2. Update/avoid the problematic browser or test environment; use a real browser with complete View Transitions support.
3. Remove/condition the `transition` NavigateOptions so navigation falls back to non-view-transition rendering.

Example fix

// before
let support = document.start_view_transition.is_some();
navigate_with_options("/next", NavigateOptions { transition: true, .. });

// after
let vt_ok = js!(return !!(document.startViewTransition &&
  document.startViewTransition(() => {}).finished);).is_truthy();
NavigateOptions { transition: vt_ok, ..Default::default() }

// Detect full View Transitions support before enabling:
let vt_supported = window()
    .and_then(|w| w.get("document").ok())
    .map(|d| d.get("startViewTransition").map(|f| f.is_function()).unwrap_or(false))
    .unwrap_or(false);

fn supports_view_transitions() -> bool {
    window().map(|w| Reflect::has(&w, &"document".into())).unwrap_or(false)
        && js!(return typeof document.startViewTransition === "function";).is_truthy()
}

// Wrap view-transition navigation:
if supports_view_transitions() {
    navigate_with_options(path, NavigateOptions { transition: true, ..Default::default() });
} else {
    navigate(path);
}

When it happens

Trigger: Navigating with `ViewTransition` enabled in a browser whose `document.startViewTransition` returns an object lacking `finished`; environments (older Safari/Firefox versions, some webviews, jsdom/test setups) with incomplete View Transitions support.

Common situations: Enabling view transitions globally and testing on a browser or headless environment without full API support; polyfills or shims that return a stub instead of a real ViewTransition; partial implementations that expose startViewTransition but not the finished promise.

Related errors

• there to be a <body> element
• there to be a <html> element
• Executor::spawn called, but no global 'spawn' function is co
• Executor::spawn_local called, but no global 'spawn_local' fu
• {location:?} expected context of type {type_name:?} to be pr

AI-assisted analysis of leptos-rs/leptos@32d20f6c9d (2026-09-01). Data as JSON: /api/errors/52ced375b77dca74. Report an issue: GitHub.