useEffect & Side Effects

Rendering should only calculate JSX. Anything that reaches outside the component is a side effect: • setting document.title • starting a timer • fetching data • subscribing to an event or a socket

useEffect runs such code after React has updated the page:

const { useEffect } = React;

useEffect(() => { document.title = "Clicked " + count + " times"; });

Without a second argument, this runs after every render.