<StrictMode> lets you find common bugs in your components early during development.

<StrictMode>
  <App />
</StrictMode>

Reference

<StrictMode>

Use StrictMode to enable additional development behaviors and warnings for the component tree inside:

import { StrictMode } from 'react';
import { createRoot } from 'react-dom/client';

const root = createRoot(document.getElementById('root'));
root.render(
  <StrictMode>
    <App />
  </StrictMode>
);

See more examples below.

Strict Mode enables the following development-only behaviors:

Props

StrictMode accepts no props.

Caveats

  • There is no way to opt out of Strict Mode inside a tree wrapped in <StrictMode>. This gives you confidence that all components inside <StrictMode> are checked. If two teams working on a product disagree whether they find the checks valuable, they need to either reach consensus or move <StrictMode> down in the tree.

Usage

Enabling Strict Mode for entire app

Strict Mode enables extra development-only checks for the entire component tree inside the <StrictMode> component. These checks help you find common bugs in your components early in the development process.

To enable Strict Mode for your entire app, wrap your root component with <StrictMode> when you render it:

import { StrictMode } from 'react';
import { createRoot } from 'react-dom/client';

const root = createRoot(document.getElementById('root'));
root.render(
  <StrictMode>
    <App />
  </StrictMode>
);

We recommend wrapping your entire app in Strict Mode, especially for newly created apps. If you use a framework that calls createRoot for you, check its documentation for how to enable Strict Mode.

Although the Strict Mode checks only run in development, they help you find bugs that already exist in your code but can be tricky to reliably reproduce in production. Strict Mode lets you fix bugs before your users report them.

補足

Strict Mode enables the following checks in development:

All of these checks are development-only and do not impact the production build.

Enabling strict mode for a part of the app

You can also enable Strict Mode for any part of your application:

import { StrictMode } from 'react';

function App() {
  return (
    <>
      <Header />
      <StrictMode>
        <main>
          <Sidebar />
          <Content />
        </main>
      </StrictMode>
      <Footer />
    </>
  );
}

In this example, Strict Mode checks will not run against the Header and Footer components. However, they will run on Sidebar and Content, as well as all of the components inside them, no matter how deep.

Fixing bugs found by double rendering in development

React assumes that every component you write is a pure function. This means that React components you write must always return the same JSX given the same inputs (props, state, and context).

Components breaking this rule behave unpredictably and cause bugs. To help you find accidentally impure code, Strict Mode calls some of your functions (only the ones that should be pure) twice in development. This includes:

If a function is pure, running it twice does not change its behavior because a pure function produces the same result every time. However, if a function is impure (for example, it mutates the data it receives), running it twice tends to be noticeable (that’s what makes it impure!) This helps you spot and fix the bug early.

Here is an example to illustrate how double rendering in Strict Mode helps you find bugs early.

This StoryTray component takes an array of stories and adds one last “Create Story” item at the end:

export default function StoryTray({ stories }) {
  const items = stories;
  items.push({ id: 'create', label: 'Create Story' });
  return (
    <ul>
      {items.map(story => (
        <li key={story.id}>
          {story.label}
        </li>
      ))}
    </ul>
  );
}

There is a mistake in the code above. However, it is easy to miss because the initial output appears correct.

This mistake will become more noticeable if the StoryTray component re-renders multiple times. For example, let’s make the StoryTray re-render with a different background color whenever you hover over it:

import { useState } from 'react';

export default function StoryTray({ stories }) {
  const [isHover, setIsHover] = useState(false);
  const items = stories;
  items.push({ id: 'create', label: 'Create Story' });
  return (
    <ul
      onPointerEnter={() => setIsHover(true)}
      onPointerLeave={() => setIsHover(false)}
      style={{
        backgroundColor: isHover ? '#ddd' : '#fff'
      }}
    >
      {items.map(story => (
        <li key={story.id}>
          {story.label}
        </li>
      ))}
    </ul>
  );
}

Notice how every time you hover over the StoryTray component, “Create Story” gets added to the list again. The intention of the code was to add it once at the end. But StoryTray directly modifies the stories array from the props. Every time StoryTray renders, it adds “Create Story” again at the end of the same array. In other words, StoryTray is not a pure function—running it multiple times produces different results.

To fix this problem, you can make a copy of the array, and modify that copy instead of the original one:

export default function StoryTray({ stories }) {
  const items = stories.slice(); // Clone the array
  // ✅ Good: Pushing into a new array
  items.push({ id: 'create', label: 'Create Story' });

This would make the StoryTray function pure. Each time it is called, it would only modify a new copy of the array, and would not affect any external objects or variables. This solves the bug, but you had to make the component re-render more often before it became obvious that something is wrong with its behavior.

In the original example, the bug wasn’t obvious. Now let’s wrap the original (buggy) code in <StrictMode>:

export default function StoryTray({ stories }) {
  const items = stories;
  items.push({ id: 'create', label: 'Create Story' });
  return (
    <ul>
      {items.map(story => (
        <li key={story.id}>
          {story.label}
        </li>
      ))}
    </ul>
  );
}

Strict Mode always calls your rendering function twice, so you can see the mistake right away (“Create Story” appears twice). This lets you notice such mistakes early in the process. When you fix your component to render in Strict Mode, you also fix many possible future production bugs like the hover functionality from before:

import { useState } from 'react';

export default function StoryTray({ stories }) {
  const [isHover, setIsHover] = useState(false);
  const items = stories.slice(); // Clone the array
  items.push({ id: 'create', label: 'Create Story' });
  return (
    <ul
      onPointerEnter={() => setIsHover(true)}
      onPointerLeave={() => setIsHover(false)}
      style={{
        backgroundColor: isHover ? '#ddd' : '#fff'
      }}
    >
      {items.map(story => (
        <li key={story.id}>
          {story.label}
        </li>
      ))}
    </ul>
  );
}

Without Strict Mode, it was easy to miss the bug until you added more re-renders. Strict Mode made the same bug appear right away. Strict Mode helps you find bugs before you push them to your team and to your users.

Read more about keeping components pure.

補足

If you have React DevTools installed, any console.log calls during the second render call will appear slightly dimmed. React DevTools also offers a setting (off by default) to suppress them completely.

Fixing bugs found by re-running Effects in development

Strict Mode can also help find bugs in Effects.

Every Effect has some setup code and may have some cleanup code. Normally, React calls setup when the component mounts (is added to the screen) and calls cleanup when the component unmounts (is removed from the screen). React then calls cleanup and setup again if its dependencies changed since the last render.

When Strict Mode is on, React will also run one extra setup+cleanup cycle in development for every Effect. This may feel surprising, but it helps reveal subtle bugs that are hard to catch manually.

Here is an example to illustrate how re-running Effects in Strict Mode helps you find bugs early.

Consider this example that connects a component to a chat:

import { useState, useEffect } from 'react';
import { createConnection } from './chat.js';

const serverUrl = 'https://localhost:1234';
const roomId = 'general';

export default function ChatRoom() {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
  }, []);
  return <h1>Welcome to the {roomId} room!</h1>;
}

There is an issue with this code, but it might not be immediately clear.

To make the issue more obvious, let’s implement a feature. In the example below, roomId is not hardcoded. Instead, the user can select the roomId that they want to connect to from a dropdown. Click “Open chat” and then select different chat rooms one by one. Keep track of the number of active connections in the console:

import { useState, useEffect } from 'react';
import { createConnection } from './chat.js';

const serverUrl = 'https://localhost:1234';

function ChatRoom({ roomId }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
  }, [roomId]);

  return <h1>Welcome to the {roomId} room!</h1>;
}

export default function App() {
  const [roomId, setRoomId] = useState('general');
  const [show, setShow] = useState(false);
  return (
    <>
      <label>
        Choose the chat room:{' '}
        <select
          value={roomId}
          onChange={e => setRoomId(e.target.value)}
        >
          <option value="general">general</option>
          <option value="travel">travel</option>
          <option value="music">music</option>
        </select>
      </label>
      <button onClick={() => setShow(!show)}>
        {show ? 'Close chat' : 'Open chat'}
      </button>
      {show && <hr />}
      {show && <ChatRoom roomId={roomId} />}
    </>
  );
}

You’ll notice that the number of open connections always keeps growing. In a real app, this would cause performance and network problems. The issue is that your Effect is missing a cleanup function:

  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => connection.disconnect();
  }, [roomId]);

Now that your Effect “cleans up” after itself and destroys the outdated connections, the leak is solved. However, notice that the problem did not become visible until you’ve added more features (the select box).

In the original example, the bug wasn’t obvious. Now let’s wrap the original (buggy) code in <StrictMode>:

import { useState, useEffect } from 'react';
import { createConnection } from './chat.js';

const serverUrl = 'https://localhost:1234';
const roomId = 'general';

export default function ChatRoom() {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
  }, []);
  return <h1>Welcome to the {roomId} room!</h1>;
}

With Strict Mode, you immediately see that there is a problem (the number of active connections jumps to 2). Strict Mode runs an extra setup+cleanup cycle for every Effect. This Effect has no cleanup logic, so it creates an extra connection but doesn’t destroy it. This is a hint that you’re missing a cleanup function.

Strict Mode lets you notice such mistakes early in the process. When you fix your Effect by adding a cleanup function in Strict Mode, you also fix many possible future production bugs like the select box from before:

import { useState, useEffect } from 'react';
import { createConnection } from './chat.js';

const serverUrl = 'https://localhost:1234';

function ChatRoom({ roomId }) {
  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.connect();
    return () => connection.disconnect();
  }, [roomId]);

  return <h1>Welcome to the {roomId} room!</h1>;
}

export default function App() {
  const [roomId, setRoomId] = useState('general');
  const [show, setShow] = useState(false);
  return (
    <>
      <label>
        Choose the chat room:{' '}
        <select
          value={roomId}
          onChange={e => setRoomId(e.target.value)}
        >
          <option value="general">general</option>
          <option value="travel">travel</option>
          <option value="music">music</option>
        </select>
      </label>
      <button onClick={() => setShow(!show)}>
        {show ? 'Close chat' : 'Open chat'}
      </button>
      {show && <hr />}
      {show && <ChatRoom roomId={roomId} />}
    </>
  );
}

Notice how the active connection count in the console doesn’t keep growing anymore.

Without Strict Mode, it was easy to miss that your Effect needed cleanup. By running setup → cleanup → setup instead of setup for your Effect in development, Strict Mode made the missing cleanup logic more noticeable.

Read more about implementing Effect cleanup.

Fixing deprecation warnings enabled by Strict Mode

React warns if some component anywhere inside a <StrictMode> tree uses one of these deprecated APIs:

These APIs are primarily used in older class components so they rarely appear in modern apps.

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