I got asked if I could write a quick explanation on the useEffect hook provided by React and thought "Sure, that should help a few people!".

useEffect can behave like componentDidMount shouldComponentUpdate and componentWillUnmount in one function if you set it up correctly. In this post I'll show you a few ways to replicate different lifecycle behaviours.

Keep in mind that useEffect uses the second argument dependencies as a performance tool

Here is an interesting read about how you can write your hooks in general even without dependencies:

https://dev.to/samsch_org/effects-are-not-lifecycles-551o

Example as componentDidMount

First you can write an Effect that will just run once when the component mounted and will never run again:

useEffect(() => {
  console.log('I was mounted and will not run again!')
}, [])

Important here is the empty array as a second argument. The second argument of useEffect can be used to watch properties for changes. See the following.

Example as shouldComponentUpdate

useEffect can also help with watchers on your properties so you can run it everytime a specific value is updated. Let's say we have a prop called "name" and our component should update something via effect everytime the name prop changes you could do it like this:

const MyComponent = (props) => {
  useEffect(() => {
    document.title = `Page of ${props.name}`
  }, [props.name])

  return <div>My name is {props.name} </div>
}

You can see that we passed props.name into the array in the second argument. This will now cause the effect to always run again when the name changes.

Side note: You should always set the second argument because otherwise you can run into render loops.

Example as componentWillUnmount

useEffect can also be used to run code when the component dismounts. This is effective for subscriptions or other listeners (Websockets for example).

let bookSubscription = null
useEffect(() => {
  // stop the subscription if it already exists
  if (bookSubscription && bookSubscription.unsubscribe) bookSubscription.unsubscribe()

  // start a new subscription
  bookSubscription = startBookSubscription({ bookId: props.bookId })

  return () => {
    // stop the subscription when the component unmounts
    bookSubscription.unsubscribe()
  }
}, [props.bookId])

You can see that now we used all options available. This code will now

1. Start a new subscription when the component was mounted
2. Update the subscription with the new bookId when the bookId prop changes
3. unsubscribe the subscription when the component gets unmounted.

You can run logic whenever the component unmounts by returning a function in your effect.

I hope this quick post was helpful to you and helps you with further development. If you have questions, let me know!