beforeunload

beforeunload is a browser event that fires on a window or frame when the document is about to be unloaded, such as during navigation to a different page, a reload, or when the user closes the tab or window. It is intended to give the page a last chance to warn the user about potential data loss.

The event is cancelable. If a listener calls preventDefault or sets a returnValue, many browsers may show a confirmation dialog asking the user to confirm leaving the page. The exact dialog text is controlled by the browser and cannot be customized by the page in modern environments.

Usage patterns typically involve checking whether the page has unsaved changes and, if so, triggering a prompt. Example usage often takes the form of registering a beforeunload listener that looks at a flag indicating unsaved work and then sets a nonempty returnValue to request confirmation.

Limitations and considerations include user experience and interoperability concerns. Because prompts can be disruptive, beforeunload should be used sparingly and only when there is meaningful potential data loss. Not all navigations will trigger a prompt, and some mobile browsers may not display one. The content of any prompt is standardized, and some browsers impose extra restrictions on when a prompt appears.

Alternatives and best practices focus on proactive data preservation. Where possible, autosave changes or use the Page Lifecycle API (with visibilitychange or pagehide) to perform saves. If a prompt is used, keep it limited to essential situations and avoid relying on it for critical functionality; consider using reliable background saving mechanisms like navigator.sendBeacon for non-blocking transmission at unload.