We will now add the possibility to specify a number of items using a text input. There are two ways to create forms in React: controlled components and uncontrolled components.
Controlled components
This is the approach that React recommends, in which form data is handled by the component rather than by the DOM. The React component is the "single source of truth".
Fix the counter-form-controlled example to use the handleChange function when the input changes.
Note that you need to bind this.handleChange to this in the constructor so you can refer to the handleChange function directly. Otherwise you would have to use onChange={event => this.handleClick(event)}, which has the unfortunate effect of creating a different callback every time the component renders.
Uncontrolled components
When you don't need to track the state in the React component, your component is said to be uncontrolled. In this case, to access the values of your form you need to use a reference to the DOM. This is done with the special ref attribute whose value is a callback that receives the DOM element as its argument.
Fix the uncontrolled counter code by ... + Number.parseInt
