the javascript method cannot work

why the form cannot suport the javascript method?

I  want  to get the  one of  the form's textbox's object in cilent-side. So I wrrtie a  stentence like this:

var  title=document.getElementById("title");

but  it produce error : "null" is null  is not an object.

there is a textbox named title in my forn. Let alone other textboxs.

why  i cannot get the object just like the same in visual studio?Is there anything I have missing?Please help me.