How to check class change automatically?

Question

JavaScript

It is necessary that all input type="text" fields constantly (or every millisecond) automatically check for the presence of the success class, and if all fields have such a class, the button color changes to red, and if not all or none of them - to grey.
The form:

<form id="post_form">
     <input id="title" class="post success">
     <input id="price" class="post">
     <input id="color" class="post invalid">
     <button>Создать</button>
    </form>

Suggest options on vue.js and jquery

Answer 1

constantly (or every millisecond) the presence of the success class was automatically checked

I think you should read more about view, especially the section on reactivity and the Observer pattern.