What is "return" in useEffect

Clean up function

There is like this code

useEffect(() => { console.log('effect execute !!') return () => { console.log('in return function') }; }, [something]); 

this function of return is called clean up function

When does clean up function execute

1. When component unmounts
2. Just Before useEffect executes

Real life example

useEffect(() => { document.addEventListener('keydown', handleKeyDown); }, [something]); 

If you write this code, keydown event adds every time when something changes. At the worst case, it causes a memory leak

so you should add clean up function, to remove event just before useEffect executes (= after state of something changes)

useEffect(() => { document.addEventListener('keydown', handleKeyDown); return () => { document.removeEventListener('keydown', handleKeyDown); }; }, [something]);