mozilla

BeforeUnloadEvent

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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.

General info

Specification
HTML5
Interface
Event
Bubbles
No
Cancelable
Yes
Target
defaultView
Default Action
Varies (prompts the user for confirmation to leave the page).

Properties

Property Type Description
target Read only EventTarget The event target (the topmost target in the DOM tree).
type Read only DOMString The type of event.
bubbles Read only boolean Does the event normally bubble?
cancelable Read only boolean Is it possible to cancel the event?
Property Description
type The type of event.
target The event target (the topmost target in the DOM tree).
currentTarget The node that had the event listener attached.

Example

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.
});

Reference

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