JSFiddle - React, Tailwind, and code Playground

HTML

<form>Username
    <br />
    <input type="text" id="user_input" name="username" />
    <br />Password
    <br />
    <input type="text" id="pass_input" name="password" />
    <br />Confirm Password
    <br />
    <input type="text" id="v_pass_input" name="v_password" />
    <br />Email
    <br />
    <input type="text" id="email" name="email" />
    <br />
    <input type="submit" id="register" value="Register" disabled="disabled" />
</form>
<div id="test"></div>
<script>
    (function() {
        $('form > input').keyup(function() {

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

            if (empty) {
                $('#register').attr('disabled', 'disabled'); // updated according to http://stackoverflow.com/questions/7637790/how-to-remove-disabled-attribute-with-jquery-ie
            } else {
                $('#register').removeAttr('disabled'); // updated according to http://stackoverflow.com/questions/7637790/how-to-remove-disabled-attribute-with-jquery-ie
            }
        });
    })();
</script>