> I have a form text field called "field_0", and javascript code:

First of all, your markup is broken. The HTML tag attributes need to be in the form of key-value pairs with the value being enclosed in double quotes.

> However if I change a little in javascript code this script will no longer work

This is because your JS code now looks for an element with the name 'fd' instead of 'field_0'. Try something like document.form1[fd].value .

But the way you are accessing forms is fundamentally broken, read this excellent article on form access.