WindowEventHandlers.onbeforeunload

The WindowEventHandlers.onbeforeunload event handler property contains the code executed when the beforeunload is sent. This event fires when a window is about to unload its resources. The document is still visible and the event is still cancelable.

Syntax

window.onbeforeunload = funcRef

• funcRef is a reference to a function or a function expression.
• The function should assign a string value to the returnValue property of the Event object and return the same string.

Example

window.onbeforeunload = function(e) {
  var dialogText = 'Dialog text here';
  e.returnValue = dialogText;
  return dialogText;
};

Notes

When this event returns an undefined value, the user is prompted to confirm the page unload. In most browsers, the return value of the event is displayed in this dialog. In Firefox 4 and later the returned string is not displayed to the user. Instead, Firefox displays the string "This page is asking you to confirm that you want to leave - data you have entered may not be saved." See bug 588292.

Since 25 May 2011, the HTML5 specification states that calls to window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event. See the HTML5 specification for more details.

Note also that various mobile browsers ignore the result of the event (that is, they do not ask the user for confirmation). Firefox has a hidden preference in about:config to do the same. In essence this means the user always confirms that the document may be unloaded.

You can and should handle this event through window.addEventListener() and the beforeunload event. More documentation is available there.

Specifications

The event was originally introduced by Microsoft in Internet Explorer 4 and standardized in the HTML5 specification.

Browser compatibility

See also

• MSDN: onbeforeunload event