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    beforeunload

    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?
    returnValue DOMString The current return value of the event (the message to show the user).

    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 and Trident
      return confirmationMessage;                                // Gecko and WebKit
    });

    Notes

    When this event returns a non-void 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.

    Reference

    Document Tags and Contributors

    Last updated by: finalfantasia,