Give your form an id only, and your input a name only:
<form id="myform">
<input type="text" name="foo">
Then the most standards-compliant and least problematic way to access your input element is via:
document.getElementById("myform").elements["foo"]
using .elements["foo"]
instead of just .foo
is preferable because the latter might return a property of the form named “foo” rather than a HTML element!