EventreturnValue

Event.returnValue, often seen as the Event.returnValue property, is a legacy, non-standard member of event objects in web browsers. Historically, it was used to control whether the default action associated with an event should proceed, and in some cases to supply a value returned by an event handler. Its use is largely deprecated in favor of the standard preventDefault() method, which explicitly cancels the default action without relying on a special property.

In practice, setting event.returnValue = false cancels the default action for many events, similar to calling event.preventDefault(). For certain events, notably onbeforeunload, assigning a non-empty string to event.returnValue could prompt the browser to show a confirmation dialog asking the user to leave the page. However, modern browsers may ignore the specific message and display a generic prompt, reflecting a shift toward standardized handling.

Compatibility and best practices: Event.returnValue remains available primarily for backward compatibility with older codebases and browsers. It is not part of the standard DOM API, and its behavior can vary between environments. For new code, developers are advised to use event.preventDefault() to cancel default actions. When dealing with beforeunload, some environments may still require setting a value on event.returnValue to trigger a prompt, but expectations about message content are unreliable and messages may be ignored.

In summary, event.returnValue is a legacy mechanism superseded by standard event.preventDefault(), with specific historical relevance to beforeunload dialogs and older browsers.