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" />
</form>

JavaScript

var $input = $('input:text'),
    $register = $('#register');

$register.attr('disabled', true);
$input.keyup(function() {
    var trigger = false;
    $input.each(function() {
        if (!$(this).val()) {
            trigger = true;
        }
    });
    trigger ? $register.attr('disabled', true) : $register.removeAttr('disabled');
});