Examples from "Making Sense of React Hooks"

MyResponsiveComponent.js

function MyResponsiveComponent() {
  const width = useWindowWidth(); // Our custom Hook
  return (
    <p>Window width is {width}</p>
  );
}

useWindowWidth.js

function useWindowWidth() {
  const [width, setWidth] = useState(window.innerWidth);
  
  useEffect(() => {
    const handleResize = () => setWidth(window.innerWidth);
    window.addEventListener('resize', handleResize);
    return () => {
      window.removeEventListener('resize', handleResize);
    };
  });
  
  return width;
}

How would one go about testing these event listeners?

Hi @gaearon, I'm very new to hooks, but isn't this typically the case where you would opt out from cleaning at each render in useEffect and rather have:

useEffect(() => {
    const handleResize = () => setWidth(window.innerWidth);
    window.addEventListener('resize', handleResize);
    return () => {
      window.removeEventListener('resize', handleResize);
    };
  }, []); // <-- empty array

As suggested in the note from the docs, the empty array makes it so that the effect is run and cleaned up when the component mounts / unmounts.

Otherwise you would add and remove a window listener at each render, right?

I had same thought, I wonder why Dan uses his original example though (useEffect without []) - it can be slightly confusing. @gaearon

i am one of the newbie in react learning.
thank you for explanation.