Components have the ability to store data that can change over time.

When a component shows data that can change over time, we want changes to be shown as soon as possible. For example, consider the ProductList component: it shows a list of products contained in the products array. If a new product is added to the array, we want it to be shown immediately. React provides a mechanism to support the automatic rendering of a component when data changes. Such a mechanism is based on the concept of state.

Again, consider the ProductList component:

import React from 'react';
import Product from './Product';

class ProductList extends React.Component {
  render() {
    let products = [
      {code:"P01", name: "Traditional Merlot", description: "A bottle
       of middle weight wine, lower in tannins (smoother), with a more...