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.
<div id="interfaceDiagram" style="display: inline-block; position: relative; width: 100%; padding-bottom: 20%; vertical-align: middle; overflow: hidden;"><svg style="display: inline-block; position: absolute; top: 0; left: 0;" viewbox="-50 0 600 120" preserveAspectRatio="xMinYMin meet"><a xlink:href="/docs/Web/API/Event" target="_top"><rect x="1" y="1" width="75" height="50" fill="#fff" stroke="#D4DDE4" stroke-width="2px" /><text x="38.5" y="30" font-size="12px" font-family="Consolas,Monaco,Andale Mono,monospace" fill="#4D4E53" text-anchor="middle" alignment-baseline="middle">Event</text></a><polyline points="76,25 86,20 86,30 76,25" stroke="#D4DDE4" fill="none"/><line x1="86" y1="25" x2="116" y2="25" stroke="#D4DDE4"/><a xlink:href="/docs/Web/API/BeforeUnloadEvent" target="_top"><rect x="116" y="1" width="170" height="50" fill="#F4F7F8" stroke="#D4DDE4" stroke-width="2px" /><text x="201" y="30" font-size="12px" font-family="Consolas,Monaco,Andale Mono,monospace" fill="#4D4E53" text-anchor="middle" alignment-baseline="middle">BeforeUnloadEvent</text></a></svg></div>
a:hover text { fill: #0095DD; pointer-events: all;}
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
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
BeforeUnloadEvent | Chrome Full support Yes | Edge Full support 12 | Firefox Full support 1.5 | IE Full support Yes | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes |
User interaction required for dialog box | Chrome Full support 60 | Edge Full support ≤79 | Firefox ? | IE ? | Opera Full support 47 | Safari ? | WebView Android Full support 60 | Chrome Android Full support 60 | Firefox Android ? | Opera Android Full support 44 | Safari iOS ? | Samsung Internet Android Full support 8.0 |
Legend
- Full support
- Full support
- Compatibility unknown
- Compatibility unknown