Rules of React's useEffect

Reacts useEffect is a powerful API with lots of capabilities, and therefore flexibility. Unfortunately, this flexibility often leads to abuse and misuse, which can greatly damage an apps stability.

The good news is that if you follow a set of rules designated to protect you during coding, your application can be secure and performant.

No, were not talking about Reacts Rules of Hooks, which includes rules such as:

• No conditionally calling hooks
• Only calling hooks inside of hooks or component
• Always having items inside of the dependency array

These rules are good, but can be detected automatically with linting rules. It's good that they're there (and maintained by Meta), but overall, we can pretend like everyone has them fixed because their IDE should throw a warning.

Specifically, I want to talk about the rules that can only be caught during manual code review processes:

• Keep all side effects inside useEffect
• Properly clean up side effects
• Don't use ref in useEffect
• Don't use [] as a guarantee that something only happens once

While these rules may seem obvious at first, we'll be taking a deep dive into the "why" of each. As a result, you may learn something about how React works under the hood - even if you're a React pro.

Keep all side effects inside useEffect

For anyone familiar with Reacts docs, youll know that this rule has been repeated over and over again. But why? Why is this a rule?

After all, what would prevent you from storing logic inside of a useMemo and simply having an empty dependency array to prevent it from running more than once?

Lets try that out by running a network request inside of a useMemo:

const EffectComp = () => {  const [activity, setActivity] = React.useState(null);  const effectFn = React.useMemo(() => {    // Make a network request here    fetch("https://www.boredapi.com/api/activity")      .then(res => res.json())      .then(res => setActivity(res.activity));  }, [])  return <p>{activity}</p>}

Run the related code sample in a sandbox environment

Huh. It works first try without any immediately noticeable downsides. This works because fetch is asynchronous, meaning that it doesnt block the event loop. Instead, lets change that code to be a synchronous XHR request and see if that works too.

function getActivity() {  var request = new XMLHttpRequest();  request.open('GET', 'https://www.boredapi.com/api/activity', false);  // `false` makes the request synchronous  request.send(null);  return JSON.parse(request.responseText);}const EffectComp = () => {  const [data, setData] = React.useState(null);  const effectFn = React.useMemo(() => {    setData(getActivity().activity);  }, []);  return <p>Hello, world! {data}</p>;}

Run the related code sample in a sandbox environment

Here, we can see behavior that we might not expect to see. When using useMemo alongside a blocking method, the entire screen will halt before drawing anything. The initial paint is then made after the fetch is finally finished.

However, if we use useEffect instead, this does not occur.

Here, we can see the initial paint occur, drawing the Hello message before the blocking network call is made.

Why does this happen?

Understanding hook lifecycles

The reason useEffect is still able to paint but useMemo cannot is because of the timings of each of these hooks. You can think of useMemo as occurring right in line with the rest of your render code.

In terms of timings, the two pieces of code are very similar:

const EffectComp = () => {  const [data, setData] = React.useState(null);  const effectFn = React.useMemo(() => {    setData(getActivity().activity);  }, []);  return <p>Hello, world! {data}</p>;}const EffectComp = () => {  const [data, setData] = React.useState(null);  setData(getActivity().activity);  return <p>Hello, world! {data}</p>;}

This inlining behavior occurs because useMemo runs during the render phase of a component. useEffect, on the other hand, runs after a component renders out, which allows an initial render before the blocking behavior halts things for us.

Those among you that know of useLayoutEffect may think you have found a gotcha in what I just said.

Ahh, but wouldnt useLayoutEffect also prevent the browser from drawing until the network call is completed?

Not quite! You see, while useMemo runs during the render phase, useLayoutEffect runs during the commit phase and therefore renders the initial contents to screen first.

useLayoutEffects signature is identical to useEffect, but it fires synchronously after all DOM mutations.

See, the commit phase is the part of a components lifecycle after React is done asking all the components what they want the UI to look like, has done all the diffing, and is ready to update the DOM.

If youd like to learn more about how React does its UI diffing and what this process all looks like under the hood, take a look at Dan Abramovs wonderful React as a UI Runtime post.

Theres also this awesome chart demonstrating how all of the hooks tie in together that our chart is a simplified version of.

Now, this isnt to say that you should optimize your code to work effectively with blocking network calls. After all, while useEffect allows you to render your code, a blocking network request still puts you in the uncomfortable position of your user being unable to interact with your page.

Because JavaScript is single-threaded, a blocking function will prevent user interaction from being processed in the event loop.

If you read the last sentence and are scratching your head, youre not alone. The idea of JavaScript being single-threaded, what an event loop is, and what blocking means are all quite confusing at first.

We suggest taking a look at this great explainer talk from Philip Robers to understand more.

That said, this isnt the only scenario where the differences between useMemo and useEffect cause misbehavior with side effects. Effectively, theyre two different tools with different usages and attempting to merge them often breaks things.

Attempting to use useMemo in place of useEffect leads to scenarios that can introduce bugs, and it may not be obvious whats going wrong at first. After long enough, with enough of these floating about in your application, its sort of death by a thousand paper-cuts.

These papercuts aren't the only problem, however. After all, the APIs for useEffect and useMemo are not the same. This incongruity between APIs is especially pronounced for network requests because a key feature is missing from the useMemo API: effect cleanup.

Always clean up your side effects

Occasionally, when using useEffect, you may be left with something that requires cleanup. A classic example of this might be a network call.

Say you have an application to give bored users an activity to do at home. Lets use a network request that retrieves an activity from an API:

const EffectComp = () => {  const [activity, setActivity] = React.useState(null);  React.useEffect(() => {    fetch("https://www.boredapi.com/api/activity")      .then(res => res.json())      .then(res => setActivity(res.activity));  }, [])  return <p>{activity}</p>}

While this works for a single activity, what happens when the user completes the activity?

Lets give them a button to rotate between new activities and include a count of how many times the user has requested an activity.

const EffectComp = () => {  const [activity, setActivity] = React.useState(null);  const [num, setNum] = React.useState(1);  React.useEffect(() => {    // Make a network request here    fetch("https://www.boredapi.com/api/activity")      .then(res => res.json())      .then(res => setActivity(res.activity));    // Re-run this effect when `num` is updated during render  }, [num])  return (  <div>    <p>You should: {activity}</p>    <p>You have done {num} activities</p>    <button onClick={() => setNum(num + 1)}>Request new activity</button>   </div>  )}

Just as we intended, we get a new network activity if we press the button. We can even press the button multiple times to get a new activity per press.

But wait, what happens if we slow down our network speed and press the request button rapidly?

Oh no! Even tho weve stopped clicking the button, our network requests are still coming in. This gives us a sluggish feeling experience, especially when latency times between network calls are high.

Well, this is where our cleanup would come into effect. Lets add an AbortSignal to cancel a request when we request a new one.

const EffectComp = () => {  const [activity, setActivity] = React.useState(null);  const [num, setNum] = React.useState(1);  React.useEffect(() => {    const controller = new AbortController();    const signal = controller.signal;    // Make a network request here    fetch("https://www.boredapi.com/api/activity", {signal})      .then(res => res.json())      .then(res => setActivity(res.activity));    return () => {      controller.abort();    }    // Re-run this effect when `num` is updated during render  }, [num])  return (  <div>    <p>You should: {activity}</p>    <p>You have done {num} activities</p>    <button onClick={() => setNum(num + 1)}>Request new activity</button>   </div>  )}

If we open our network request tab, youll notice how our network calls are now being canceled when we initialize a new one.

This is a good thing! It means that instead of a jarring experience of jumpiness, youll now only see a single activity after the end of a chain of clicking.

While this may seem like a one-off that we created ourselves using artificial network slowdowns, this is the real-world experience users on slow networks may experience!

Whats more, when you consider API timing differences, this problem may be even more widespread.

Lets say that youre using a new React concurrent feature, which may cause an interrupted render, forcing a new network call before the other has finished.

The first call hangs on the server for slightly longer for whatever reason and takes 500ms, but the second call goes through immediately in 20ms. But oh no, during that 480ms there was a change in the data!

This means that our .then which runs setActivity will execute on the first network call complete with stale data (showing 10,000) after the second network call.

This is important to catch early, because these shifts in behavior can be immediately noticeable to a user when it happens. These issues are also often particularly difficult to find and work through after the fact.

Dont use refs in useEffect

If youve ever used a useEffect to apply an addEventListener, you may have written something like the following:

const RefEffectComp = () => {  const buttonRef = React.useRef();  const [count, setCount] = React.useState(0);  React.useEffect(() => {    function buttonAdder() {        setCount(v => v + 1);    }    buttonRef.current.addEventListener('click', buttonAdder);    return () => {        buttonRef.current.removeEventListener('click', buttonAdder);        }  }, [buttonRef.current])  return <div>    <p>{count}</p>    <button ref={buttonRef}>Click me</button>  </div>}

Run the related code sample in a sandbox environment

While this might make intuitive sense due to utilizing useEffects cleanup, this code is actually not correct. You should not utilize a ref or ref.current inside of a dependency array for a hook.

This is because changing refs does not force a re-render and therefore useEffect never runs when the value changes.

While most assume that useEffect listens for changes in this array and runs the effect when it changes, this is an inaccurate mental model.

A more apt mental model might be: useEffect only runs at most once per render. However, as an optimization, I can pass an array to prevent the side effect from running if the variable references inside of the array have not changed.

This shift in understanding is important because the first version can easily lead to bugs in your app. For example, instead of rendering out the button immediately, lets say that we need to defer the rendering for some reason.

Simple enough, well add a setTimeout and a boolean to render the button.

const RefEffectComp = ()=>{  const buttonRef = React.useRef();  const [count, setCount] = React.useState(0);  React.useEffect(() => {    function buttonAdder() {      setCount(v => v + 1);    }          console.log('UseEffect has run');          // This will throw an error during the first render otherwise    if (!buttonRef.current) return;    buttonRef.current.addEventListener('click', buttonAdder);    return () => {      buttonRef.current.removeEventListener('click', buttonAdder);        }  }, [buttonRef.current])  const [shouldRender, setShouldRender] = React.useState(false);  React.useEffect(() => {    const timer = setTimeout(() => {      setShouldRender(true);    }, 1000);    return () => {      clearTimeout(timer);      setShouldRender(false);    }  }, []);  return <div>    <p>{count}</p>    {shouldRender && <button ref={buttonRef}>Click me</button>}  </div>}

Run the related code sample in a sandbox environment

Now, if we wait a second for the button to render and click it, our counter doesnt go up!

This is because once our ref is set after the initial render, it doesnt trigger a re-render and our useEffect never runs.

A better way to write this would be to utilize a callback ref, and then use a useState to force a re-render when its set.

const RefEffectComp = ()=>{  const [buttonEl, setButtonEl] = React.useState();  const [count, setCount] = React.useState(0);  React.useEffect(() => {    function buttonAdder() {      setCount(v => v + 1);    }    if (!buttonEl) return;    buttonEl.addEventListener('click', buttonAdder);    return () => {      buttonEl.removeEventListener('click', buttonAdder);        }  }, [buttonEl])  const [shouldRender, setShouldRender] = React.useState(false);  React.useEffect(() => {    const timer = setTimeout(() => {      setShouldRender(true);    }, 1000);    return () => {      clearTimeout(timer);      setShouldRender(false);    }  }, []);  return <div>    <p>{count}</p>    {shouldRender && <button ref={buttonElRef => setButtonEl(buttonElRef)}>Click me</button>}  </div>}

This will force the re-render when ref is set after the initial render and, in turn, cause the useEffect to trigger as expected.

To be fair, this rule is more of a soft rule than anything. There are absolutely instances - such as setTimeout timers - where utilizing a ref inside of a useEffect make sense. Just make sure you have a proper mental model about refs and useEffect and youll be fine.

Want to refine your understanding of refs even further? See my article outlining the important details of refs for more.

Dont expect an empty dependency array to only run once

While previous versions of React allowed you to utilize an empty array to guarantee that a useEffect would only run once, React 18 changed this behavior. As a result, now useEffect may run any number of times when an empty dependency array passes, in particular when a concurrent feature is utilized.

Concurrent features are new to React 18 and allow React to pause, halt, and remount a component whenever React sees it appropriate.

As a result, this may break various aspects of your code.

You can read more about how an empty dependency array can break in your app from our article about React 18s changes to mounting.

Conclusion

Reacts useEffect is an instrumental part of modern React applications. Now that you know more about its internals and the rules around it, you can build stronger and more dynamic programs!

If you want to continue learning skills that will help make your React apps better, I suggest taking a look at our guide to React Unidirectionality, which outlines a good way to keep your application flow more organized.