Hooks are a new addition in React They let you use state and other React features without writing a class. This snippet is based on the counter example from the previous pagebut we added a new feature to it: we set the document title to a custom message including the number of clicks.

Data fetching, setting up a subscription, and manually changing the DOM in React components are all examples of side effects.

Sometimes, we want to run some additional code after React has updated the DOM. We say that because we can run them and immediately forget about them. It would be too early — we typically want to perform our effects after React has updated the DOM. Coming back to our example, here is a React counter class component that updates the document title right after React makes changes to the DOM:. Note how we have to duplicate the code between these two lifecycle methods in class.

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This is because in many cases we want to perform the same side effect regardless of whether the component just mounted, or if it has been updated. We could extract a separate method but we would still have to call it in two places.

What does useEffect do? By using this Hook, you tell React that your component needs to do something after render. In this effect, we set the document title, but we could also perform data fetching or call some other imperative API.

Why is useEffect called inside a component? Placing useEffect inside the component lets us access the count state variable or any props right from the effect. Does useEffect run after every render?

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By default, it runs both after the first render and after every update. We will later talk about how to customize this. React guarantees the DOM has been updated by the time it runs the effects. We declare the count state variable, and then we tell React we need to use an effect.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I can not understand why the following useImperativeHandle, useLayoutEffect, and useDebugValue hooks are needed, can you give examples when they can be used, but not examples from the documentation please.

Allow me to preface this answer by stating that all of these hooks are very rarely used. They are only meant to cover some rare corner-case scenarios. Usually when you use useRef you are given the instance value of the component the ref is attached to. This allows you to interact with the DOM element directly.

There could be many reasons you want might to do either of the above; you might not want to expose native properties to the parent or maybe you want to change the behavior of a native function.


There could be many reasons. However, useImperativeHandle is rarely used. In this example, the value we'll get from the ref will only contain the function blur which we declared in our useImperativeHandle. It will not contain any other properties I am logging the value to demonstrate this. The function itself is also "customized" to behave differently than what you'd normally expect. Here, it sets document. While similar to some extent to useEffectit differs in that it will run after React has committed updates to the DOM.

The signature is identical to useEffectbut it fires synchronously after all DOM mutations. Use this to read layout from the DOM and synchronously re-render. Updates scheduled inside useLayoutEffect will be flushed synchronously, before the browser has a chance to paint. Suppose you have a absolutely positioned element whose height might vary and you want to position another div beneath it.

You could use getBoundingCLientRect to calculate the parent's height and top properties and then just apply those to the top property of the child. Here you would want to use useLayoutEffect rather than useEffect. See why in the examples below:. Sometimes you might want to debug certain values or properties, but doing so might require expensive operations which might impact performance.

I have personally never used this hook though. Maybe someone in the comments can give some insight with a good example. Imagine you create a component Table. You may want to specify how refs on Table interact with that component; it may be that, for instance, you want to provide a particular cell, rather than only being able to access the root element. Available here.

How the useEffect Hook Works

This is the same as useEffectbut only fires once all DOM mutations are completed. This article From Kent C. Dodds explains the difference as well as anyone, regarding these two, he says:. I haven't seen any examples which illustrate this particularly well, and I'm not sure I'd be able to create anything either. It's probably best to say that you ought to only use useLayoutEffect when useEffect has issues.

useEffect vs. useLayoutEffect in plain, approachable language

I feel like the docs do a pretty good example of explaining this one.Photo by Mohamed Nohassi on Unsplash. Both of these can be used to do basically the same thing, but they have slightly different use cases.

So here are some rules for you to consider when deciding which React Hook to use. When hooks are stable and if you refactor any of your class components to use hooks, you'll likely move any code from componentDidMountcomponentDidUpdateand componentWillUnmount to useEffect. The one catch is that this runs after react renders your component and ensures that your effect callback does not block browser painting. This differs from the behavior in class components where componentDidMount and componentDidUpdate run synchronously after rendering.

It's more performant this way and most of the time this is what you want. You'll want to use useLayoutEffect. Otherwise the user could see a flicker when your DOM mutations take effect. This is pretty much the only time you want to avoid useEffect and use useLayoutEffect instead.

This runs synchronously immediately after React has performed all DOM mutations. This can be useful if you need to make DOM measurements like getting the scroll position or other styles for an element and then make DOM mutations or trigger a synchronous re-render by updating state. As far as scheduling, this works the same way as componentDidMount and componentDidUpdate.

Your code runs immediately after the DOM has been updated, but before the browser has had a chance to "paint" those changes the user doesn't actually see the updates until after the browser has repainted. I am extremely excited about React's upcoming hooks feature. I think it's going to make React much easier to learn and use. Kent C. Dodds is a JavaScript software engineer and teacher. He's taught hundreds of thousands of people how to make the world a better place with quality software development tools and practices.

He lives with his wife and four kids in Utah.Picture this: you have a perfectly good function component, and then one day, you need to add a lifecycle method to it. Cue the class Thing extends React. Componentand copy-pasting the function body into renderand then fixing the indentation, and then finally adding the lifecycle method.


With useEffectyou can handle lifecycle events directly inside function components. All with one function! Crazy, I know. Look at the console. Now, try the Re-render button. That seems weird…. This is actually more powerful than the componentWillUnmount lifecycle because it lets you run a side effect before and after every render, if you need to.

If you want your effects to run less often, you can provide a second argument — an array of values. Think of them as the dependencies for that effect. If one of the dependencies has changed since the last time, the effect will run again. It will also still run after the initial render. Another way to think of this array: it should contain every variable that the effect function uses from the surrounding scope.

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So if it uses a prop? That goes in the array.

Hooks API Reference

If it uses a piece of state? So if we changed our component above to call useEffect like this:. This comes with a big warning, though: passing the empty array is prone to bugs. Sometimes you just want to do one tiny thing at mount time, and doing that one little thing requires rewriting a function as a class.

To prove it, try out the example. Then try typing in the box. This is bad. Passing no 2nd argument causes the useEffect to run every render.

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Then, when it runs, it fetches the data and updates the state. Then, once the state is updated, the component re-renders, which triggers the useEffect again. You can see the problem.

The only variable that useEffect depends on is setPosts. Therefore we should pass the array [setPosts] here. Fun fact: When you call useStatethe setter function it returns is only created once!

This fun fact is also true for the dispatch function returned by useReducer. This is still hard-coded, but now we can customize it by wrapping the Reddit component with one that lets us change the subreddit. Add this new App component, and render it at the bottom:.

The app is keeping 2 pieces of state here — the current input value, and the current subreddit. Wrapping the input in a form allows the user to press Enter to submit. Implementing error handling would be a great exercise though!If you use server rendering, keep in mind that neither useLayoutEffect nor useEffect can run until the JavaScript is downloaded. You might see a warning if you try to useLayoutEffect on the server.

Here's two common ways to fix it. If this effect isn't important for first render i. If UI looks broken with useEffect but gets fixed by useLayoutEffectit means that this component doesn't look right until the effect runs. So server-rendering it brings no benefit and shows a confusing UI. To fix this, you can delay showing that component until after the client side JS loads and hydrates the component.

To exclude a Child that needs layout effects from the server-rendered HTML, you can render it conditionally:. If you have some use case that isn't covered, please report a complete minimal code example here and we'll try to help.

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Is there any way to silence the warning? I have a use case where the layout-effect is only applied when a prop changes away from it's initial server-rendered value.

So the hypothetical "won't look right" doesn't apply and I'd rather not needlessly refactor my components only to appease an over-eager alarm-system. Yeah what maranomynet said. It's so annoying I keep seeing it in my test logs. Warning: useLayoutEffect does nothing on the server, because its effect cannot be encoded into the server renderer's output format.

This will lead to a mismatch between the initial, non-hydrated UI and the intended UI. To avoid this, useLayoutEffect should only be used in components that render exclusively on the client. The warning useLayoutEffect does nothing on the server is a common problem during testing when using Redux and Jest. The warning is triggered because react-redux uses useLayoutEffect instead of useEffect when window is defined, and Jest defines window by default.

To fix the warning, select the node test environment, e. Or, to select the node environment globally, use this in your package. The warning about useLayoutEffect should now disappear because window is no longer defined during test execution. A little bit hackish workaround for the problem if you still require the mocked window or cannot change the testEnvironment.

I had to put this in the setup file and it worked.


This problem might be fixed in react-redux 7. It seems like you would need to add a new State to SomeComponent based on the value of contentTransition to go around this warning. I have a use case in which I need to detect the browser's viewport dimensions and basing on it, rerender the component if needed. I'd like to do it synchronously just after the component is mounted in class component, I'd do it with componentDidMount.


This is because desktop user shouldn't see mobile version of that component and vice-versa. I don't want to delay it to an asynchronous effect as useEffect would do.

Shouldn't there be a way to silence this warning gaearon? After all I imagine you don't know the viewport size on the server, or do you infer that from user agent?

In that case the warning is quite correct Warning: useLayoutEffect does nothing on the server.

React Hooks: Advanced Hooks

I feel the real question is how to re-run your viewport resize code when SSR html is rehydrated, isn't it? MHwishes have you found a way to fix it?There are two React hooks, useEffect and useLayoutEffectthat appear to work pretty much the same. Read on for what makes them different and when to use each. That goes:. The right time to useLayoutEffect instead? Literally. Just wanted to mention it! Try the useLayoutEffect version and then try the version with useEffect.

Notice how the version with useLayoutEffect only updates visually once even though the component rendered twice. The useEffect version, on the other hand, visually renders twice, so you see a flicker where the value is briefly 0. Most of the time, useEffect is the right choice. If your code is causing flickering, switch to useLayoutEffect and see if that helps. Because useLayoutEffect is synchronous a.

Learning React can be a struggle — so many libraries and tools! My advice? Ignore all of them : For a step-by-step approach, check out my Pure React workshop. Hats off. I'm a React trainer in London and would thoroughly recommend this to all front end devs wanting to upskill or consolidate.

The way you call them even looks the same. Alan Lavender. Now check your email. Finally understand how React works! There was an error submitting your subscription. Please try again. Email Address.August 8, 8 min read Assuming you really understand the difference between useEffect and useLayoutEffectcan you explain this difference in simple terms? Perhaps the most prominent of these is found in the first paragraph detailing the useLayoutEffect Hook:.

The first clause in the sentence above is easy to understand. The signature for both Hooks are identical. The signature for useEffect is shown below:. In fact, if you go through a codebase and replace every useEffect call with useLayoutEffectalthough different, this will work in most cases. However, the second part of the aforementioned quote above feels a little fuzzy to most people:. The difference between useEffect and useLayoutEffect is solely when they are fired.

With every click of the button, the counter state is updated, the DOM mutation printed to the screen, and the effect function triggered. The user performs an action, i. React updates the count state variable internally. With the click comes a state update, which in turn triggers a DOM mutation, i. Steps 1, 2, and 3 above do not show any visual change to the user.

React hands over the details about the DOM mutation to the browser engine, which figures out the entire process of painting the change to the screen. Understanding the next step is crucial to the discussed subject. Only after the browser has painted the DOM change s is the useEffect function fired. What to note here is that the function passed to useEffect will be fired only after the DOM changes are painted to the screen.

Technically speaking, the effect function is fired asynchronously not to block the browser paint process. If the useEffect function itself triggers another DOM mutation, this happens after the first, but the process is usually pretty fast.

It triggers the function right after the DOM mutations are computed. While this seems unideal, it is highly encouraged in specific use cases. For example, a DOM mutation that must be visible to the user should be fired synchronously before the next paint. This is so that the user does not perceive a visual inconsistency.

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Remember, updates scheduled inside useLayoutEffect will be flushed synchronously, before the browser has a chance to paint. As stated in the sections above, the difference between useEffect and useLayoutEffect is in when they are fired.

Modern browsers are fast — very fast. We will employ some creativity to see how the time of execution differs between useEffect and useLayoutEffect.

Note that the effects log different texts depending on which is triggered, and as expected, the first effect function is triggered before the second. When there are more than one useEffect calls within a component, the order of the effect calls is maintained.

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The first is triggered, then the second — on and on the sequence goes. This is understandable. However, the useEffect function is called after the DOM mutations are painted. The example app has a button that toggles the visual state of a title — whether shaking or not. The reason I chose this example is to make sure the browser actually has some fun changes to paint when the button is clicked, hence the animation.