Understanding useEffect design

React provides a useEffect hook to set up a callback to be invoked after an update:

const Title = () => {
  useEffect(() => {
    window.title = "Hello World"
    return () => {
      window.title = "Notitle"
    }
  }, [])
}

The useEffect function takes a callback function called create as its first input argument to define the effect. In the preceding example, the effect sets the window.title to be Hello World when the component is mounted.

A create function can return a function called destroy to perform the cleanup. The interesting thing here is that the destroy function is provided by the create function as a return value. In the preceding example, the cleanup reverts the window.title object back to NoTitle when it is unmounted.

The second parameter in the useEffect argument list is a dependency array...