React recently introduced a new way to deal with side effects: the useEffect hook. Translating lifecycle methods to useEffect calls can be confusing at first. It’s confusing because we shouldn’t be translating imperative lifecycle methods to declarative useEffect calls in the first place.

It’s easy to forget to “clean up” a side effect when a component unmounts, or update the side effect when props change. As Dan Abramov preaches: Don’t stop the data flow .

React components have always relied on lifecycle methods for side effects. While lifecycle methods get the job done, they’re often overly verbose and have large margins for error.

Listening to websockets

Consider a ChatChannel component that listens to websockets.

import React , { Component } from 'react' ; import websockets from 'websockets' ; class ChatChannel extends Component { state = { messages : []; } componentDidMount () { websockets . listen ( `channels. ${ this . props . channelId } ` , message => { this . setState ( state => { return { messages : [... state . messages , message ] }; }); } ); } render () { // ... } }

If we run this in our browser, it will work! However, there are a two silent issues.

First, we never clean up our side effect. When the ChatChannel component unmounts, the websocket listener is still registered. When a new event comes in, the callback will run and will try to update the state of a component that doesn’t exist anymore. We need to implement a componentWillUnmount method to clean things up.

import React , { Component } from 'react' ; import websockets from 'websockets' ; class ChatChannel extends Component { state = { messages : []; } componentDidMount () { websockets . listen ( `channels. ${ this . props . channelId } ` , message => { this . setState ( state => { return { messages : [... state . messages , message ] }; }); } ); } componentWillUnmount () { websockets . unlisten ( `channels. ${ this . props . channelId } ` ); } render () { // ... } }

We’ll also be seeing some very weird chat threads when the channelId prop changes over time. The component would render a different channel, but the initial channelId 's events will still be pushed through.

To solve this we add a componentDidUpdate implementation and shuffle things around to keep things DRY.

import React , { Component } from 'react' ; import websockets from 'websockets' ; class ChatChannel extends Component { state = { messages : []; } componentDidMount () { this . startListeningToChannel ( this . props . channelId ); } componentDidUpdate ( prevProps ) { if ( this . props . channelId !== prevProps . channelId ) { this . stopListeningToChannel ( prevProps . channelId ); this . startListeningToChannel ( this . props . channelId ); } } componentWillUnmount () { this . stopListeningToChannel ( this . props . channelId ); } startListeningToChannel ( channelId ) { websockets . listen ( `channels. ${ channelId } ` , message => { this . setState ( state => { return { messages : [... state . messages , message ] }; }); } ); } stopListeningToChannel ( channelId ) { websockets . unlisten ( `channels. ${ channelId } ` ); } render () { // ... } }

The problem with lifecycle methods for side effects is that they run based on implementation details of the way React renders components. In practice, side effects are based on what truly matters: our data that can change over time. In this case, props.

Refactoring to useEffect

Let’s rewrite our ChatChannel component with useEffect .

import React , { useEffect , useState } from 'react' ; import websockets from 'websockets' ; function ChatChannel ({ channelId }) { const [ messages , setMessages ] = useState ([]); useEffect (() => { websockets . listen ( `channels. ${ channelId } ` , message => setMessages ( messages => [... messages , message ]) ); return () => websockets . unlisten ( `channels. ${ channelId } ` ); }, [ channelId ]); // ... }

Instead of hooking into the component lifecycle’s nitty gritty details, we simply declare that there’s some listen side effect based on channelId 's current value. We also return a unlisten call in a closure. React will run that cleanup callback whenever the effect is invalidated: either when channelId changes, or when the component unmounts.

Instead of thinking about when we should apply the side effect, we declare the side effect’s dependencies. This way React knows when it needs to run, update, or clean up.

That’s where the power of useEffect lies. The websocket listener doesn’t care about mounting and unmounting components, it cares about the value of channelId over time.