JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <input type="text" />
    <input type="password" />
    <input type="email" />
    <input type="text" />
    <input type="text" />
    <textarea></textarea>
    <select>
        <option selected disabled hidden value=''></option>
        <option>Option1</option>
        <option>Option2</option>
    </select>
    <select>
        <option selected disabled hidden value=''></option>
        <option>Option1</option>
        <option>Option2</option>
    </select>
    <select>
        <option selected disabled hidden value=''></option>
        <option>Option1</option>
        <option>Option2</option>
    </select>
    <input type="submit" id="register" value="Register"/>
</form>

CSS

input, textarea, select{
    display: block;
    margin-top: 5px;
}
body{
    padding: 15px;
}

JavaScript

$('#register').attr('disabled', 'disabled');
(function() {
    $('form > input, form > textarea, form > select').change(function(){
        var empty = false;
        $('form > input, form > textarea, form > select').each(function() {
            if ($(this).val() == '') {
                empty = true;
            }
        });
        if (empty) {
            $('#register').attr('disabled', 'disabled');
        } else {
            $('#register').removeAttr('disabled');
        }
    });
})()