The beforeunload event is fired when the window, the document and its resources are about to be unloaded.
When a non-empty string is assigned to the returnValue Event property, a dialog box appears, asking the users for confirmation to leave the page (see example below). When no value is provided, the event is processed silently.
| Bubbles | No |
| Cancelable | Yes |
| Target objects | defaultView |
| Interface | Event |
Examples
window.addEventListener("beforeunload", function( event ) {
event.returnValue = "\o/";
});
//is equivalent to
window.addEventListener("beforeunload", function( event ) {
event.preventDefault();
});
Webkit-based browsers don't follow the spec for the dialog box. An almost cross-browser working example would be close to the below example.
window.addEventListener("beforeunload", function (e) {
var confirmationMessage = "\o/";
(e || window.event).returnValue = confirmationMessage; //Gecko + IE
return confirmationMessage; //Webkit, Safari, Chrome etc.
});