JSFiddle - React, Tailwind, and code Playground

HTML

<form id="myform">Username
    <br />
    <input type="text" id="user_input" name="username" />
    <br />Password
    <br />
    <input type="password" id="pass_input" name="password" />
    <br />Confirm Password
    <br />
    <input type="password" id="v_pass_input" name="v_password" />
    <br />Email
    <br />
    <input type="text" id="email" name="email" />
    <br />Birthday
    <br />
    <input type="date" id="bday" name="birthday" />
    <br />Sex
    <br />
    <select name="sex" id="sex">
        <option>Male</option>
        <option>female</option>
    </select>
    <input type="submit" id="register" value="Register" disabled="disabled" />
</form>

JavaScript

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