jQuery中的多个beforeunload处理程序

Question

I am writing JavaScript for a plugin that will be used on clients' sites, and need to use a beforeunload handler to store some information to the user's cookies or localstorage for tracking purposes.

However, since I don't control the clients' sites, I'm concerned about the scenario where the client's page has already defined a beforeunload handler before we set ours. In particular, I'm worried about the case where the client's JavaScript creates a 'beforeunload' handler that returns a string to create an 'Are you sure you want to leave this page?' -style popup, and the possibility that adding our handler after that may destroy or otherwise interfere with this functionality.

Based upon a little experimentation in the console in Chromium, it seems that if I add handlers using jQuery(window).on('beforeunload', handler) , then all the handlers get executed in the order they were added and then the existence and content of any confirmation dialog is determined solely by the final non- undefined return value . So if the last handler added with an explicit return returns null , no message is shown, and if it returns a string, then a confirmation dialog with that string as its content is shown. This is the behaviour I want; it means that as long as I return undefined from the beforeunload handler that I add, I shouldn't break any of our clients' code.

Can I rely upon this behaviour across all browsers, though? How about if the client added their original beforeunload handler via a mechanism other than jQuery, like explicitly assigning to window.onbeforeunload or using window.addEventListener ? How is jQuery handling the assignment of multiple handlers behind the scenes?

Answer 1

what if you do it like this

 var x = window.onbeforeunload
 window.onbeforeunload = function() { 
       // do your stuf
       if (x) return x();
 }

so this way you'll return whatever the initial event handler was suppose to return.