We used to pass a parent component's method as a prop to the child component like this:
<SomeComponent onClick={this._click.bind(this)} />
But according to the video I watched, it actually creates a reference in memory on that binded function each time React renders the component. And it's so long that we need to type .bind(this)
every time we pass a method.
So they introduced a way to pre-bind the method in the constructor
like this: