JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" action="" id="myForm">
<label for="login">Логин</label><input type="text" name="login" id="login">
<label for="pass">Пароль</label><input type="password" name="pass" id="pass">
<button id="button" type="submit" disabled="true">Вход</button>
</form>

JavaScript

var form = $('#myForm'),
    login = $('#login'),
    pass = $('#pass'),
    submit = $('#button');
form.change(function(){
    if(login[0].value != '' && pass[0].value != ''){
        button.disabled = false;
    }else{
        button.disabled = true;
    }
})