React hooks: how to detect when particular state variable has updated [duplicate]

Question

Before React hooks, I would use componentDidUpdate(prevProps, prevState), and if I wanted to execute given code only when this.state.a had updated, I'd do

if (prevState.a !== this.state.a) {
  <...>
}

How can I achieve the same thing in useEffect()?

Answer 1

The useEffect function takes a dependency array where you can mention the states you want to track for change.

useEffect(() => {
    //your code here
}, [a])