Why does useEffect ignore the last element of the array?

Question

I am learning React, especially learning useEffect.

What I can't figure out is why useEffect skips the last element of the array.

Can someone kindly explain why and how to fix it?

let users = ['Oliver', 'Thomas', 'George', 'William']
export default function App() {
  const [index, setIndex] = useState(0);
  console.log('RENDER');
  useEffect(() => {
    if (index === users.length - 1) {
      return
    }
    setTimeout(() => setIndex(index => index 1), 2000)
    console.log('Hello ' users[index]);
    console.log('Side Effect RUNS!');
  }, [users[index]])

}

Answer 1

Your useEffect will not run because users[index] has the same value

Change it to index

let users = ['Oliver', 'Thomas', 'George', 'William']
export default function App() {
  const [index, setIndex] = useState(0);
  console.log('RENDER');
  useEffect(() => {
    if (index === users.length - 1) {
      return
    }
    setTimeout(() => setIndex(index => index + 1), 2000)
    console.log('Hello ' + users[index]);
    console.log('Side Effect RUNS!');
  }, [index]) // 更改为 `[index]`

}