All browsers implement an event named beforeunload Show archive.org snapshot . It is fired when the active window is closed and can be used to display an alert to warn the user about unsaved changes.

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To trigger the alert, you have to call preventDefault() on the event.

Note

The beforeunload event is only dispatched when the user navigation makes a full page load, or if it closes the tab entirely. It will not be dispatched when navigating via JavaScript. In this case you need to listen to other events.

Quirks

• The "default behavior" in this case is not to show an alert, and preventing this default means the alert is being shown.
• In ancient browsers you could customize the alert text, but this is no longer possible.
• You might want to set event.returnValue = true, as a truthy returnValue was necessary to trigger the alert in Chrome < 119
• You might want to return any string from the event callback as this was the mechanism to trigger the alert in older browsers
• You cannot use the debugger statement within the event callback

Example

// Assuming you've implemented a "isFormDirty" function
function warnBeforeUnload (event) {
  if (isFormDirty()) {
    event.preventDefault() // show standard alert with it's default text
    const leaveMessage = 'Leave page? Changes you made may not be saved.'
    event.returnValue = leaveMessage // support for Chrome < 119
    return leaveMessage // support for older browsers
  }
}

window.addEventListener('beforeunload', warnBeforeUnload)