useEffectフックと依存配列
useEffectが実行されるタイミングを理解し、依存配列で制御して、クリーンアップ関数を返すことで購読やタイマーを解除します
「useEffectフックと依存配列」はCoddyKit上の無料React Native Academyレッスンです。 これはレッスン1/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはReact Native Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 React Native Academyコースには全4レッスンが含まれています。
このレッスンの一部はまだ翻訳されておらず、英語で表示されています。
What is useEffect?
useEffect lets function components perform side effects: fetching data, setting up subscriptions, manipulating the DOM, starting timers, or calling native APIs. Side effects cannot be run directly in the render function because renders must be pure. useEffect runs after the component renders to the screen.
import { useEffect } from 'react';
function App() {
useEffect(() => {
console.log('Component rendered');
// Side effects go here
});
}The Dependency Array
The second argument to useEffect is the dependency array. React uses it to decide when to re-run the effect. If the dependency array is omitted, the effect runs after every render. If it is empty ([]), the effect runs once after the initial render. If it contains values, the effect re-runs whenever any of those values change.
// Runs after every render
useEffect(() => { ... });
// Runs once on mount
useEffect(() => { ... }, []);
// Runs on mount and whenever userId changes
useEffect(() => { ... }, [userId]);Fetching Data on Mount
The most common useEffect pattern is fetching data when a component first mounts. Pass an empty dependency array to ensure the fetch runs exactly once. Store the result in state and render it in the JSX. React calls the effect after the initial paint, so the component renders once with empty/loading state before the data arrives.
function PostsScreen() {
const [posts, setPosts] = useState([]);
useEffect(() => {
fetch('https://jsonplaceholder.typicode.com/posts')
.then((res) => res.json())
.then((data) => setPosts(data));
}, []); // empty array = run once on mount
return <FlatList data={posts} ... />;
}Reactive Dependencies
When you want the effect to re-run whenever a value changes (like a search query or a user ID), list that value in the dependency array. React compares each dependency with its previous value after every render, and if any have changed, it re-runs the effect. This creates a reactive data flow: UI event → state update → effect re-runs.
function UserProfile({ userId }) {
const [user, setUser] = useState(null);
useEffect(() => {
fetch('/api/users/' + userId)
.then((res) => res.json())
.then((data) => setUser(data));
}, [userId]); // re-fetch whenever userId changes
if (!user) return <ActivityIndicator />;
return <Text>{user.name}</Text>;
}The Cleanup Function
Return a function from your effect to perform cleanup. React calls this cleanup function before re-running the effect (on dependency changes) and when the component unmounts. Use it to cancel subscriptions, clear timers, or abort fetch requests to prevent memory leaks and state updates on unmounted components.
useEffect(() => {
const timer = setInterval(() => {
setTime(Date.now());
}, 1000);
return () => clearInterval(timer); // cleanup on unmount or re-run
}, []);Cancelling Fetch Requests with AbortController
When a component unmounts before a fetch completes, React will warn about updating state on an unmounted component. Use the AbortController to cancel the in-flight fetch in the cleanup function. Catching the abort error (which is an AbortError) prevents it from being logged as an unexpected error.
useEffect(() => {
const controller = new AbortController();
fetch('/api/data', { signal: controller.signal })
.then((res) => res.json())
.then((data) => setData(data))
.catch((err) => {
if (err.name !== 'AbortError') setError(err);
});
return () => controller.abort();
}, []);Running Code Only on Unmount
To run code only when the component unmounts (like unsubscribing from an event listener), return a cleanup function from a useEffect with an empty dependency array. React calls the cleanup only when the component leaves the tree, never on subsequent renders.
useEffect(() => {
const subscription = eventEmitter.addListener('change', handler);
return () => {
subscription.remove(); // only on unmount
};
}, []); // empty array = mount-only effectMultiple useEffect Calls
You can and should use multiple useEffect calls in one component — one for each independent side effect. React runs them in the order they appear after every render. Keeping effects separated by concern makes the code easier to read and avoids complex dependency arrays that mix unrelated logic.
// Effect 1: fetch user data
useEffect(() => {
fetchUser(userId).then(setUser);
}, [userId]);
// Effect 2: analytics page view
useEffect(() => {
analytics.logScreenView('Profile');
}, []);
// Effect 3: sync title to header
useEffect(() => {
navigation.setOptions({ title: user?.name ?? 'Profile' });
}, [user, navigation]);Dependency Pitfalls: Missing Dependencies
A common bug is omitting a dependency from the array. The effect captures a stale value of the missing dependency and runs with outdated data. Always include every value from the component scope that is read inside the effect. ESLint's react-hooks/exhaustive-deps rule catches these automatically.
// Bug: fetchUser uses userId but it is not in deps
// If userId changes, the effect still runs with the old userId
useEffect(() => {
fetchUser(userId).then(setUser);
}, []); // Missing userId!
// Correct:
useEffect(() => {
fetchUser(userId).then(setUser);
}, [userId]);Avoiding Infinite Loops
If your effect updates state that is also listed as a dependency, you get an infinite loop: render → effect → state update → render → effect → .... Guard against this with a condition inside the effect, or restructure so the effect does not trigger the state update that caused it to run.
// Infinite loop: effect updates 'count' which is in deps
useEffect(() => {
setCount(count + 1); // triggers re-render → effect runs again
}, [count]);
// Fix: use functional updater — no need to list count as dep
useEffect(() => {
setCount((c) => c + 1);
}, []); // only runs onceComparing Effects to Lifecycle Methods
useEffect replaces all three classic class component lifecycle methods: componentDidMount (empty deps array), componentDidUpdate (with deps), and componentWillUnmount (cleanup function). One hook handles all three phases, keeping related logic together rather than spread across three separate methods.
useEffect(() => {
// componentDidMount equivalent — fetch initial data
fetchData();
return () => {
// componentWillUnmount equivalent — cancel subscriptions
cancelSubscription();
};
}, []); // runs once on mount, cleanup on unmountQuick Check
Test your understanding of the useEffect hook and dependency arrays from this lesson.
Lesson Recap
In this lesson you learned: useEffect runs side effects after render, the dependency array controls when the effect re-runs — empty means once on mount, and return a cleanup function to cancel subscriptions, timers, and fetch requests on unmount. Next up we make GET requests with Axios inside useEffect.
よくある質問
「useEffectフックと依存配列」レッスンは無料ですか？
はい。「useEffectフックと依存配列」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、React Native Academyコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 React Native Academyコースには全4レッスンが含まれています。
「useEffectフックと依存配列」で何を学びますか？
useEffectが実行されるタイミングを理解し、依存配列で制御して、クリーンアップ関数を返すことで購読やタイマーを解除します ブラウザで直接実行するハンズオンコードでReact Native Academyを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。
React Native Academyを始めるのに経験は必要ですか？
事前経験は必要ありません。CoddyKitのReact Native Academyは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン1/4です。
「useEffectフックと依存配列」レッスンにはどのくらい時間がかかりますか？
ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。
このReact Native Academyレッスンでコードを書いて実行できますか？
はい。すべてのReact Native Academyレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。
このコースのすべてのレッスン
- useEffectフックと依存配列
- AxiosによるGETリクエスト
- ローディング状態とエラー状態の処理
- AxiosによるPOST、PUT、DELETE