Definition and Usage
The onbeforeunload event occurs when the document is about to be unloaded.
This event allows you to display a message in a confirmation dialog box to inform the user whether he/she wants to stay or leave the current page.
The default message that appears in the confirmation box, is different in different browsers. However, the standard message is something like "Are you sure you want to leave this page?". This message cannot be removed.
However, you can write a custom message together with the default message. See the first example on this page. Note: This only works in Internet Explorer.
Note: If the onbeforeunload event is not assigned to the <body> element, you must assign/attach the event on the window object, and use the returnValue property to create a custom message (see syntax examples below).
The numbers in the table specify the first browser version that fully supports the event.
Note: The addEventListener() method is not supported in Internet Explorer 8 and earlier versions.
|Event type:||UiEvent if generated from a user interface, Event otherwise|
|Supported HTML tags:||<body>|
|DOM Version:||Level 2 Events|