useEffect(() => {

  fetch(`https://jsonplaceholder.typicode.com/${query}`)

    .then(response => response.json())

    .then(json => setData(json));

}, [query]); // only re-run on query change

const App = (props) => {

  const [change, setChange] = useState(false);

​

  useEffect(() => {

    return () => console.log("CHANGED!");

  }, [change]);

​

  return (

    <div>

      <button onClick={() => setChange(true)}>CLICK</button>

    </div>

  );

};

useEffect(() => {

        // send HTTP request

        // save response to variable

    }, [])

What does useEffect do? By using this Hook, you tell React that 

your component needs to do something after render. 

React will remember the function you passed (we’ll refer to it 

as our “effect”), and call it later after performing the DOM updates.