Synchronizing with effects Flashcards
Synchronizing with effects
Some components need to synchronize with external systems. For example, you might want to control a non-React component based on the React state, set up a server connection, or send an analytics log when a component appears on the screen.
Effects let you run some code after rendering so that you can synchronize your component with some system outside of React.
What are Effects and how are they different from events?
Before getting to Effects, you need to be familiar with two types of logic inside React components:
- Rendering code (introduced in Describing the UI) lives at the top level of your component. This is where you take the props and state, transform them, and return the JSX you want to see on the screen. Rendering code must be pure. Like a math formula, it should only calculate the result, but not do anything else.
- Event handlers (introduced in Adding Interactivity) are nested functions inside your components that do things rather than just calculate them. An event handler might update an input field, submit an HTTP POST request to buy a product, or navigate the user to another screen. Event handlers contain “side effects” (they change the program’s state) and are caused by a specific user action (for example, a button click or typing).
Sometimes this isn’t enough. Consider a ChatRoom component that must connect to the chat server whenever it’s visible on the screen. Connecting to a server is not a pure calculation (it’s a side effect) so it can’t happen during rendering. However, there is no single particular event like a click that causes ChatRoom to be displayed.
Effects let you specify side effects that are caused by rendering itself, rather than by a particular event. Sending a message in the chat is an event because it is directly caused by the user clicking a specific button. However, setting up a server connection is an Effect because it needs to happen regardless of which interaction caused the component to appear.
Effects run at the end of the rendering process after the screen updates. This is a good time to synchronize the React components with some external system (like network or a third-party library).
You might not need an Effect
Don’t rush to add Effects to your components. Keep in mind that Effects are typically used to “step out” of your React code and synchronize with some external system. This includes browser APIs, third-party widgets, network, and so on. If your effect only adjusts some state based on other state, you might not need an Effect.
How to write an effect
To write an Effect, follow these three steps:
- Declare an Effect. By default, your Effect will run after every render.
- Specify the Effect dependencies. Most Effects should only re-run when needed rather than after every render. For example, a fade-in animation should only trigger when a component appears. Connecting and disconnecting to a chat room should only happen when the component appears and disappears, or when the chat room changes. You will learn how to control this by specifying dependencies.
- Add cleanup if needed. Some Effects need to specify how to stop, undo, or clean up whatever they were doing. For example, “connect” needs “disconnect,” “subscribe” needs “unsubscribe,” and “fetch” needs either “cancel” or “ignore”. You will learn how to do this by returning a cleanup function.
Step 1: Declare an Effect
To declare an Effect in your component, import the useEffect Hook from React:
Then, call it at the top level of your component and put some code inside your Effect
Every time your component renders, React will update the screen and then run the code inside useEffect. In other words, useEffect “delays” a piece of code from running until that render is reflected on the screen.
Let’s see how you can use an Effect to synchronize with an external system. Consider a React component. It would be nice to control whether it’s playing or paused by passing an isPlaying prop to it:
;
Your custom VideoPlayer component renders the built-in browser tag:
function VideoPlayer({ src, isPlaying }) {
// TODO: do something with isPlaying
return ;
}
However, the browser tag does not have an isPlaying prop. The only way to control it is to manually call the play() and pause() methods on the DOM element. You need to synchronize the value of isPlaying prop, which tells whether the video should currently be playing, with imperative calls like play() and pause().
We’ll need to first get a ref to the DOM node.
The reason this code isn’t correct is that it tries to do something with the DOM node during rendering. In React, rendering should be a pure calculation of JSX and should not contain side effects like modifying the DOM.
Moreover, when VideoPlayer is called for the first time, its DOM does not exist yet! There isn’t a DOM node yet to call play() or pause() on, because React doesn’t know what DOM to create until after you return the JSX.
The solution here is to wrap the side effect with useEffect to move it out of the rendering calculation.
By wrapping the DOM update in an Effect, you let React update the screen first. Then your Effect runs.
When your VideoPlayer component renders (either the first time or if it re-renders), a few things will happen. First, React will update the screen, ensuring the tag is in the DOM with the right props. Then React will run your Effect. Finally, your Effect will call play() or pause() depending on the value of isPlaying prop.
In this example, the “external system” you synchronized to React state was the browser media API. You can use a similar approach to wrap legacy non-React code (like jQuery plugins) into declarative React components.
Infinite loop with useEffect
By default, Effects run after every render. This is why code like this will produce an infinite loop:
const [count, setCount] = useState(0);
useEffect(() => {
setCount(count + 1);
});
Effects run as a result of rendering. Setting state triggers rendering. Setting state immediately in an Effect is like plugging a power outlet into itself. The Effect runs, it sets the state, which causes a re-render, which causes the Effect to run, it sets the state again, this causes another re-render, and so on.
Effects should usually synchronize your components with an external system. If there’s no external system and you only want to adjust some state based on other state, you might not need an Effect.
Step 2: Specify the Effect dependencies
By default, Effects run after every render. Often, this is not what you want:
Sometimes, it’s slow. Synchronizing with an external system is not always instant, so you might want to skip doing it unless it’s necessary. For example, you don’t want to reconnect to the chat server on every keystroke.
Sometimes, it’s wrong. For example, you don’t want to trigger a component fade-in animation on every keystroke. The animation should only play once when the component appears for the first time.
You can tell React to skip unnecessarily re-running the Effect by specifying an array of dependencies as the second argument to the useEffect call. Start by adding an empty [] array
You should see an error saying React Hook useEffect has a missing dependency: ‘isPlaying’.
The problem is that the code inside of your Effect depends on the isPlaying prop to decide what to do, but this dependency was not explicitly declared. To fix this issue, add isPlaying to the dependency array.
Now all dependencies are declared, so there is no error. Specifying [isPlaying] as the dependency array tells React that it should skip re-running your Effect if isPlaying is the same as it was during the previous render.
The dependency array can contain multiple dependencies. React will only skip re-running the Effect if all of the dependencies you specify have exactly the same values as they had during the previous render. React compares the dependency values using the Object.is comparison.
Notice that you can’t “choose” your dependencies. You will get a lint error if the dependencies you specified don’t match what React expects based on the code inside your Effect. This helps catch many bugs in your code. If your Effect uses some value but you don’t want to re-run the Effect when it changes, you’ll need to edit the Effect code itself to not “need” that dependency.
Why was the ref omitted from the dependency array?
This is because the ref object has a stable identity: React guarantees you’ll always get the same object from the same useRef call on every render. It never changes, so it will never by itself cause the Effect to re-run. Therefore, it does not matter whether you include it or not.
The set functions returned by useState also have stable identity, so you will often see them omitted from the dependencies too. If the linter lets you omit a dependency without errors, it is safe to do.
Omitting always-stable dependencies only works when the linter can “see” that the object is stable. For example, if ref was passed from a parent component, you would have to specify it in the dependency array. However, this is good because you can’t know whether the parent component always passes the same ref, or passes one of several refs conditionally. So your Effect would depend on which ref is passed.
Step 3: Add cleanup if needed
Consider a different example. You’re writing a ChatRoom component that needs to connect to the chat server when it appears. You are given a createConnection() API that returns an object with connect() and disconnect() methods. How do you keep the component connected while it is displayed to the user?
Start by writing the Effect logic.
It would be slow to connect to the chat after every re-render, so you add the dependency array.
The code inside the Effect does not use any props or state, so your dependency array is [] (empty). This tells React to only run this code when the component “mounts,” i.e. appears on the screen for the first time.
Let’s try running this code:
import { useState, useEffect } from ‘react’;
import { createConnection } from ‘./chat.js’;
export default function ChatRoom() {
useEffect(() => {
const connection = createConnection();
connection.connect();
}, []);
return <h1>Welcome to the chat!</h1>;
}
This Effect only runs on mount, so you might expect “Connecting…” to be printed once in the console. However, if you check the console, “Connecting…” gets printed twice. Why does it happen?
Imagine the ChatRoom component is a part of a larger app with many different screens. The user starts their journey on the ChatRoom page. The component mounts and calls connection.connect(). Then imagine the user navigates to another screen—for example, to the Settings page. The ChatRoom component unmounts. Finally, the user clicks Back and ChatRoom mounts again. This would set up a second connection—but the first connection was never destroyed! As the user navigates across the app, the connections would keep piling up.
To fix the issue, return a cleanup function from your Effect:
React will call your cleanup function each time before the Effect runs again, and one final time when the component unmounts (gets removed).
React re-mounts ever component twice in development
To help you spot bugs quickly in development React remounts every component once immediately after its initial mount. Seeing the “Connecting…” log twice helps you notice the real issue: your code doesn’t close the connection when the component unmounts.
This is the correct behavior in development. By remounting your component, React verifies that navigating away and back would not break your code. Disconnecting and then connecting again is exactly what should happen! When you implement the cleanup well, there should be no user-visible difference between running the Effect once vs running it, cleaning it up, and running it again. There’s an extra connect/disconnect call pair because React is probing your code for bugs in development. This is normal and you shouldn’t try to make it go away.
In production, you would only see “Connecting…” printed once. Remounting components only happens in development to help you find Effects that need cleanup. You can turn off Strict Mode to opt out of the development behavior, but we recommend keeping it on. This lets you find many bugs like the one above.
How to handle the Effect firing twice in development?
React intentionally remounts your components in development to help you find bugs like in the last example. The right question isn’t “how to run an Effect once,” but “how to fix my Effect so that it works after remounting”.
Usually, the answer is to implement the cleanup function. The cleanup function should stop or undo whatever the Effect was doing. The rule of thumb is that the user shouldn’t be able to distinguish between the Effect running once (as in production) and an effect → cleanup → effect sequence (as you’d see in development).
Most of the Effects you’ll write will fit into one of these common patterns
- Controlling non-React widgets
Sometimes you need to add UI widgets that aren’t written to React. For example, let’s say you’re adding a map component to your page. It has a setZoomLevel() method, and you’d like to keep the zoom level in sync with a zoomLevel state variable in your React code. Your Effect would look like similar to this:
useEffect(() => {
const map = mapRef.current;
map.setZoomLevel(zoomLevel);
}, [zoomLevel]);
Note that there is no cleanup needed in this case. In development, React will call the Effect twice, but this is not a problem because calling setZoomLevel twice with the same value does not do anything. It may be slightly slower, but this doesn’t matter because the remounting is development-only and won’t happen in production.
Some APIs may not allow you to call them twice in a row. For example, the showModal method of the built-in element throws if you call it twice. Implement the cleanup function and make it close the dialog:
useEffect(() => {
const dialog = dialogRef.current;
dialog.showModal();
return () => dialog.close();
}, []);
- Subscribing to events
If your Effect subscribes to something, the cleanup function should unsubscribe:
useEffect(() => {
function handleScroll(e) {
console.log(e.clientX, e.clientY);
}
window.addEventListener('scroll', handleScroll);
return () => window.removeEventListener('scroll', handleScroll);
}, []);
In development, your Effect will call addEventListener(), then immediately removeEventListener(), and then addEventListener() again with the same handler. So there would be only one active subscription at a time. This has the same user-visible behavior as calling addEventListener() once, as you would see in production.
- Triggering animations
If your Effect animates something in, the cleanup function should reset the animation to the initial values:
useEffect(() => {
const node = ref.current;
node.style.opacity = 1; // Trigger the animation
return () => {
node.style.opacity = 0; // Reset to the initial value
};
}, []);
In development, opacity will be set to 1, then to 0, and then to 1 again. This should have the same user-visible behavior as setting it to 1 directly, which is what would happen in production. If you use a third-party animation library with support for tweening, your cleanup function should reset the tween’s timeline to its initial state.
In development, opacity will be set to 1, then to 0, and then to 1 again. This should have the same user-visible behavior as setting it to 1 directly, which is what would happen in production. If you use a third-party animation library with support for tweening, your cleanup function should reset the tween’s timeline to its initial state.
- Fetching data
If your Effect fetches something, the cleanup function should either abort the fetch or ignore its result:
useEffect(() => {
let ignore = false;
async function startFetching() {
const json = await fetchTodos(userId);
if (!ignore) {
setTodos(json);
}
}
startFetching();
return () => {
ignore = true;
};
}, [userId]);
You can’t “undo” a network request that already happened, but your cleanup function should ensure that the fetch that’s not relevant anymore does not keep affecting your application. For example, if the userId changes from ‘Alice’ to ‘Bob’, cleanup ensures that the ‘Alice’ response is ignored even if it arrives after ‘Bob’.
In development, you will see two fetches in the Network tab. There is nothing wrong with that. With the approach above, the first Effect will immediately get cleaned up so its copy of the ignore variable will be set to true. So even though there is an extra request, it won’t affect the state thanks to the if (!ignore) check.
In production, there will only be one request. If the second request in development is bothering you, the best approach is to use a solution that deduplicates requests and caches their responses between components:
This will not only improve the development experience, but also make your application feel faster. For example, the user pressing the Back button won’t have to wait for some data to load again because it will be cached. You can either build such a cache yourself or use one of the many existing alternatives to manual fetching in Effects.
- Sending analytics
Consider this code that sends an analytics event on the page visit:
useEffect(() => {
logVisit(url); // Sends a POST request
}, [url]);
In development, logVisit will be called twice for every URL, so you might be tempted to try to work around it. We recommend to keep this code as is. Like with earlier examples, there is no user-visible behavior difference between running it once and running it twice. From a practical point of view, logVisit should not do anything in development because you don’t want the logs from the development machines to skew the production metrics. Your component remounts every time you save its file, so it would send extra visits during development anyway.
In production, there will be no duplicate visit logs.
To debug the analytics events you’re sending, you can deploy your app to a staging environment (which runs in production mode) or temporarily opt out of Strict Mode and its development-only remounting checks. You may also send analytics from the route change event handlers instead of Effects. For even more precise analytics, intersection observers can help track which components are in the viewport and how long they remain visible.
What are good alternatives to data fetching in Effects?
Writing fetch calls inside Effects is a popular way to fetch data, especially in fully client-side apps. This is, however, a very manual approach and it has significant downsides:
- Effects don’t run on the server. This means that the initial server-rendered HTML will only include a loading state with no data. The client computer will have to download all JavaScript and render your app only to discover that now it needs to load the data. This is not very efficient.
- Fetching directly in Effects makes it easy to create “network waterfalls”. You render the parent component, it fetches some data, renders the child components, and then they start fetching their data. If the network is not very fast, this is significantly slower than fetching all data in parallel.
- Fetching directly in Effects usually means you don’t preload or cache data. For example, if the component unmounts and then mounts again, it would have to fetch the data again.
- It’s not very ergonomic. There’s quite a bit of boilerplate code involved when writing fetch calls in a way that doesn’t suffer from bugs like race conditions.
This list of downsides is not specific to React. It applies to fetching data on mount with any library. Like with routing, data fetching is not trivial to do well, so we recommend the following approaches:
- If you use a framework, use its built-in data fetching mechanism. Modern React frameworks have integrated data fetching mechanisms that are efficient and don’t suffer from the above pitfalls.
- Otherwise, consider using or building a client-side cache. Popular open source solutions include React Query, useSWR, and React Router 6.4+. You can build your own solution too, in which case you would use Effects under the hood but also add logic for deduplicating requests, caching responses, and avoiding network waterfalls (by preloading data or hoisting data requirements to routes).
You can continue fetching data directly in Effects if neither of these approaches suit you.
Not effects
- Not an Effect: Initializing the application
Some logic should only run once when the application starts. You can put it outside your components:
if (typeof window !== ‘undefined’) { // Check if we’re running in the browser.
checkAuthToken();
loadDataFromLocalStorage();
}
function App() {
// ...
}
This guarantees that such logic only runs once after the
browser loads the page.
- Not an Effect: Buying a product
Sometimes, even if you write a cleanup function, there’s no way to prevent user-visible consequences of running the Effect twice. For example, maybe your Effect sends a POST request like buying a product:
useEffect(() => {
// 🔴 Wrong: This Effect fires twice in development, exposing a problem in the code.
fetch('/api/buy', { method: 'POST' });
}, []);
You wouldn’t want to buy the product twice. However, this is also why you shouldn’t put this logic in an Effect. What if the user goes to another page and then presses Back? Your Effect would run again. You don’t want to buy the product when the user visits a page; you want to buy it when the user clicks the Buy button.
Buying is not caused by rendering; it’s caused by a specific interaction. It only runs once because the interaction (a click) happens once. Delete the Effect and move your /api/buy request into the Buy button event handler:
function handleClick() {
// ✅ Buying is an event because it is caused by a particular interaction.
fetch('/api/buy', { method: 'POST' });
}
This illustrates that if remounting breaks the logic of your application, this usually uncovers existing bugs. From the user’s perspective, visiting a page shouldn’t be different from visiting it, clicking a link, and then pressing Back. React verifies that your components don’t break this principle by remounting them once in development.
Effects summary
- Unlike events, Effects are caused by rendering itself rather than a particular interaction.
- Effects let you synchronize a component with some external system (third-party API, network, etc).
- By default, Effects run after every render (including the initial one).
- React will skip the Effect if all of its dependencies have the same values as during the last render.
- You can’t “choose” your dependencies. They are determined by the code inside the Effect.
- An empty dependency array ([]) corresponds to the component “mounting”, i.e. being added to the screen.
- When Strict Mode is on, React mounts components twice (in development only!) to stress-test your Effects.
- If your Effect breaks because of remounting, you need to implement a cleanup function.
- React will call your cleanup function before the Effect runs next time, and during the unmount.
