hashchange

Hashchange is a browser event that fires when the fragment identifier of the current URL changes. The fragment identifier is the portion of the URL that follows a hash symbol (#). This event enables web applications to respond to in-page navigation without triggering a full page reload, supporting lightweight routing and client-side state changes.

The event is triggered when the hash portion changes to a different value. This can occur when a user clicks a link that points to a new hash, when a script updates location.hash, or when the history mechanism alters the fragment. It does not fire for changes to the path, query string, or other parts of the URL unless the hash itself changes. The event fires after the new URL is determined.

The event object, available to the handler, typically exposes the oldURL and newURL properties, allowing code to determine what changed. In practice, developers often read location.hash within the handler to determine the current fragment and map it to a view or state.

Usage commonly involves attaching a listener to the hashchange event, for example using window.addEventListener('hashchange', handler). In the handler, code usually reads location.hash and updates the visible content accordingly. This pattern is popular in single-page applications that implement routing without server round-trips.

Browser support is broad: the event is supported in modern browsers and has been available since older versions of Internet Explorer (IE 8 and later). Hashchange remains a simple, de facto standard for in-page navigation, though it is complemented by the History API (pushState and replaceState) for more flexible URL management.