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    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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    Contributors to this page: rabimba
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