<p>beforeunload happend before unload. beforeunload can give user a message, asking user if want to contine or not, if yes, unload event will trigger and unload can show user a message, but can not block to unload the page. Below is the sample.</p>
<pre data-sub="prettyprint:_">
// Attach to the beforeunload handler
window.onbeforeunload = function(){
// Return an explanation for why the user should not leave the page.
return 'Your data will not be saved.';
};
// Watch for the user leaving the web site
window.onunload = function(){
// Display a message to the user, thanking them for visiting
alert( 'Thanks for visiting!' );
};
</pre>