An observable state

The state is at the epicenter of all things happening in the UI. MobX provides a core building block, called the observable, that represents the reactive state of your application. Any JavaScript object can be used to create an observable. We can use the aptly named observable() API as follows:

import {observable} from 'mobx';

let cart = observable({
    itemCount: 0,
    modified: new Date()
});

In the preceding example, we have created a simple cart object that is also an observable. The observable() API comes from the mobx NPM package. With this simple declaration of an observable, we now have a reactive cart that keeps track of changes happening on any of its properties: itemCount and modified.