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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. Some implementations only show the dialog box if the frame or any embedded frame receives a user gesture or user interaction. See Browser compatibility for more information.

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-derived 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;                                /* Safari, Chrome, and other
                                                              * WebKit-derived browsers */
});

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'BeforeUnloadEvent' in that specification.
Living Standard Initial definition

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
BeforeUnloadEventChrome Full support YesEdge Full support 12Firefox Full support 1.5IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yes
User interaction required for dialog boxChrome Full support 60Edge Full support ≤79Firefox ? IE ? Opera Full support 47Safari ? WebView Android Full support 60Chrome Android Full support 60Firefox Android ? Opera Android Full support 44Safari iOS ? Samsung Internet Android Full support 8.0

Legend

Full support  
Full support
Compatibility unknown  
Compatibility unknown

See also

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最近更新: neoncitylights,