Final answer:
The JavaScript event handler that is used to detect when a user is about to close a browser window is the 'beforeunload' event. It can trigger a confirmation dialog to ask the user if they want to leave the page. The functionality is limited due to security and usability reasons.
Step-by-step explanation:
JavaScript Event Handler for Browser Window Close
The event handler in JavaScript that detects when a user is about to close a browser window or tab is the beforeunload event. This event fires when a window is about to unload its resources, which typically happens when a window or tab is being closed or when the user is navigating away from the page. It is important to note that the actual functionality that can be implemented with the beforeunload event is limited due to security and usability reasons.
To use the beforeunload event, you would typically attach an event listener to the window object. For example:
window.addEventListener('beforeunload', function(event) {
// Your code here
});
The event does not provide a standard way to prevent the closing of the window, but it can be used to trigger a confirmation dialog asking the user if they really want to leave the page. The return value of the event handler determines the behavior of this confirmation dialog. The handling of beforeunload may vary between different browsers, and developers need to test across different environments to ensure consistent behavior.