I want my form submit button to be disabled/enabled depending on if the form is completely filled.

When the inputs are filled, the disabled button changes to enabled. That works great.
But I would like it to disable the button when an input gets emtied.

This is my script:

<script type=text/javascript language=javascript>

    function checkform()

    {

        var f = document.forms[theform].elements;

        var cansubmit = true;



        for (var i = 0; i < f.length; i++) {

            if (f[i].value.length == 0) cansubmit = false;

        }



        if (cansubmit) {

            document.getElementById('submitbutton').disabled = false;

        }

    }

</script> 

<form name=theform>

<input type=text onKeyup=checkform() />

<input type=text onKeyup=checkform() />

<input id=submitbutton type=submit disabled=disabled value=Submit />

</form>