componentDidMount equivalent on a React function/Hooks component?

For the stable version of hooks (React Version 16.8.0+)

For componentDidMount

useEffect(() => {
  // Your code here
}, []);

For componentDidUpdate

useEffect(() => {
  // Your code here
}, [yourDependency]);

For componentWillUnmount

useEffect(() => {
  // componentWillUnmount
  return () => {
     // Your code here
  }
}, [yourDependency]);

So in this situation, you need to pass your dependency into this array. Let's assume you have a state like this

const [count, setCount] = useState(0);

And whenever count increases you want to re-render your function component. Then your useEffect should look like this

useEffect(() => {
  // <div>{count}</div>
}, [count]);

This way whenever your count updates your component will re-render. Hopefully this will help a bit.

There is no exact equivalent for componentDidMount in react hooks.

---

In my experience, react hooks requires a different mindset when developing it and generally speaking you should not compare it to the class methods like componentDidMount.

With that said, there are ways in which you can use hooks to produce a similar effect to componentDidMount.

Solution 1:

useEffect(() => {
  console.log("I have been mounted")
}, [])

Solution 2:

const num = 5

useEffect(() => {
  console.log("I will only run if my deps change: ", num)
}, [num])

Solution 3 (With function):

useEffect(() => {
  const someFunc = () => {
    console.log("Function being run after/on mount")
  }
  someFunc()
}, [])

Solution 4 (useCallback):

const msg = "some message"

const myFunc = useCallback(() => {
  console.log(msg)
}, [msg])

useEffect(() => {
  myFunc()
}, [myFunc])

Solution 5 (Getting creative):

export default function useDidMountHook(callback) {
  const didMount = useRef(null)

  useEffect(() => {
    if (callback && !didMount.current) {
      didMount.current = true
      callback()
    }
  })
}

It is worth noting that solution 5 should only really be used if none of the other solutions work for your use case. If you do decide you need solution 5 then I recommend using this pre-made hook use-did-mount.

Source (With more detail): Using componentDidMount in react hooks