disable submit button until form is filled

HTML

<form>
    Username<br />
    <input type="text" id="user_input" name="username" /><br />
    Password<br />
    <input type="password" id="pass_input" name="password" /><br />
   
    <input type="submit" id="sign_in" value="submit" disabled="disabled" />
</form>
<div id="test">
</div>

JavaScript

(function() {
    $('form > input').keyup(function() {

        var empty = false;
        $('form > input').each(function() {
            if ($(this).val() == '') {
                empty = true;
            }
        });

        if (empty) {
            $('#submit').attr('disabled', 'disabled');
        } else {
            $('#sign_in').removeAttr('disabled');
        }
    });
})()

/* instead of form we can use the IDs of certain fields only. In case not all fields are mandatory */