Let's say you want to trap the window close event for the web browser so that you can give a confirmation dialog asking if the user is sure to leave the page. The problem is that there is no onclose event for the window object. The closest event might be onunload since it fires immediately before the window object is unloaded. However, when the onunload event fires it is too late to display a JavaScript alert. Therefore, we need an event that fires prior to a page being unloaded, which is onbeforeunload. Define onbeforeunload event in your page <BODY> element as follows:
<BODY onbeforeunload="HandleOnClose()">
Then, add the following JavaScript code into the <HEAD> section of your ASPX page:
<script language="javascript">
<!--
function HandleOnClose() {
if (event.clientY < 0) {
event.returnValue = 'Are you sure you want to leave the page?';
}
}
//-->
</script>
The trick here is to check clientY property of the event object, which is used to set or retrieve the y-coordinate of the mouse pointer's position relative to the client area of the window, excluding window decorations and scroll bars. This way, you can detect if the user clicked on X button to close the page, or clicked on Refresh button to refresh the page, etc. This approach does not handle key events such as Alt-F4 that lets the user close the window by using the keyboard. You have to handle keyboard events separately and this issue might be a topic for another article...
Famil Jones,
Karamasoft