Disable/Enable Submit Button until all forms have been filled

Question

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>

Answer 1

Just use

document.getElementById('submitbutton').disabled = !cansubmit;

instead of the the if-clause that works only one-way.

Also, for the users who have JS disabled, I'd suggest to set the initial disabled by JS only. To do so, just move the script behind the <form> and call checkform(); once.