I needed this today! While developing a custom web part for Sharepoint 2013, I discovered there are some quirks when writing client-side javascript.
http://chris.gg/2010/02/sharepoint-and-getelementbyid/
Instead of:
document.getElementById(“TextBox1”);
Use this:
document.getElementById(“<%=TextBox1.ClientID %>”);
It’s a bit of a pain to have to do this. But, it works.