Question

I want to intercept any postbacks in the current page BEFORE it occurs . I want to do some custom manipulation before a postback is served. Any ideas how to do that?

Answer 1

There’s a couple of things you can do to intercept a postback on the client.

The __doPostBack function looks like this:

function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}

Notice that it calls “theForm.onsubmit()” before actually doing the postback. This means that if you assign your form an onsubmit javascript function, it will always be called before every postback.

<form id="form1" runat="server" onsubmit="return myFunction()">

Alternately, you can actually override the __doPostBack function and replace it with your own. This is an old trick that was used back in ASP.Net 1.0 days.

var __original= __doPostBack;
__doPostBack = myFunction();

This replaces the __doPostBack function with your own, and you can call the original from your new one.