Fix: ReactDOM.render is not a function - React 18 Complete Migration Guide

The ‘ReactDOM.render is not a function’ error is a common issue developers encounter when upgrading to React 18 or when working with modern React applications. This error occurs because React 18 introduced a new React Root API that replaced the legacy ReactDOM.render() method with createRoot().render().

This comprehensive guide explains what changed in React 18, why this error occurs, and provides multiple solutions to fix it in your React applications with clean code examples and directory structure.

---

What Changed in React 18?

React 18 introduced a new Concurrent Rendering system and a new React Root API. The legacy ReactDOM.render() method was deprecated and replaced with createRoot().render() for better performance, automatic batching, and new features like automatic state updates batching.

Key Changes:

• Legacy API: ReactDOM.render(element, container)
• New API: root.render(element)
• Automatic batching: Better performance for state updates
• Concurrent features: New rendering capabilities

---

Common Error Messages:

• ReactDOM.render is not a function
• TypeError: ReactDOM.render is not a function
• React 18: ReactDOM.render is no longer supported

---

Understanding the Problem

In React 17 and earlier, applications were typically bootstrapped using:

// Legacy React 17 approach
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root')
);

However, in React 18, ReactDOM.render was removed from the react-dom package and moved to react-dom/client.

Typical React Project Structure:

my-react-app/
├── package.json
├── src/
│   ├── index.js (or main.jsx for Vite)
│   ├── App.jsx
│   └── components/
│       └── MyComponent.jsx
└── public/
    └── index.html

---

Solution 1: Update to React 18 Root API (Recommended)

Step 1: Update index.js (or main.jsx for Vite)

Before (React 17):

// src/index.js
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root')
);

After (React 18):

// src/index.js
import React from 'react';
import { createRoot } from 'react-dom/client';
import './index.css';
import App from './App';

const container = document.getElementById('root');
const root = createRoot(container);

root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

Step 2: Update Package Dependencies

Ensure you have React 18+ installed:

npm install react@latest react-dom@latest

Updated package.json:

{
  "name": "my-react-app",
  "version": "0.1.0",
  "dependencies": {
    "react": "^18.2.0",
    "react-dom": "^18.2.0"
  }
}

---

Solution 2: Using React DOM Client Import

For Create-React-App Projects:

// src/index.js
import React from 'react';
import { createRoot } from 'react-dom/client';
import './index.css';
import App from './App';

const rootElement = document.getElementById('root');
const root = createRoot(rootElement);

root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

For TypeScript Projects:

// src/index.tsx
import React from 'react';
import { createRoot } from 'react-dom/client';
import './index.css';
import App from './App';

const container = document.getElementById('root');
if (!container) throw new Error('Failed to find the root element');

const root = createRoot(container);

root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

---

Solution 3: Vite Project Configuration

For Vite-based React projects, update the main entry point:

Before (Vite with React 17):

// src/main.jsx
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(<App />, document.getElementById('root'));

After (Vite with React 18):

// src/main.jsx
import React from 'react';
import { createRoot } from 'react-dom/client';
import App from './App';

const container = document.getElementById('root');
const root = createRoot(container);

root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

---

Solution 4: Next.js Project Update

For Next.js projects, update your _app.js or _app.jsx:

Before (Next.js with React 17):

// pages/_app.js
import '../styles/globals.css';

function MyApp({ Component, pageProps }) {
  return <Component {...pageProps} />;
}

export default MyApp;

After (Next.js with React 18):

// pages/_app.js
import '../styles/globals.css';

function MyApp({ Component, pageProps }) {
  return <Component {...pageProps} />;
}

export default MyApp;

Note: Next.js handles the root API internally, so you typically don’t need to change the client-side rendering in _app.js.

---

Solution 5: Conditional Rendering for Migration

If you’re migrating gradually, you can create a compatibility function:

// src/compatibility.js
import { createRoot } from 'react-dom/client';
import ReactDOM from 'react-dom';

export function render(element, container) {
  if (createRoot) {
    // React 18
    const root = createRoot(container);
    root.render(element);
  } else {
    // React 17 and below
    ReactDOM.render(element, container);
  }
}

// Usage
import { render } from './compatibility';
import App from './App';

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

---

Solution 6: Testing Environment Updates

Update your testing setup for React 18:

Jest Setup:

// src/setupTests.js
import '@testing-library/jest-dom';

// For React 18, ensure proper cleanup
import { act } from 'react-dom/test-utils';

// Cleanup after each test
afterEach(() => {
  act(() => {
    // Any cleanup needed
  });
});

Testing with React 18:

// src/App.test.js
import { render, screen } from '@testing-library/react';
import App from './App';

test('renders learn react link', () => {
  render(<App />);
  const linkElement = screen.getByText(/learn react/i);
  expect(linkElement).toBeInTheDocument();
});

---

Solution 7: Server-Side Rendering (SSR) Updates

For SSR applications, update your server-side rendering:

Before (React 17 SSR):

// server.js
import React from 'react';
import ReactDOMServer from 'react-dom/server';
import App from './App';

const html = ReactDOMServer.renderToString(<App />);

After (React 18 SSR):

// server.js
import React from 'react';
import { renderToString } from 'react-dom/server';
import App from './App';

const html = renderToString(<App />);

Note: Server-side rendering APIs remain the same in React 18.

---

Complete Project Structure After Migration

Standard React App:

my-react-app/
├── package.json
├── package-lock.json
├── node_modules/
├── public/
│   └── index.html
├── src/
│   ├── index.js
│   ├── App.jsx
│   ├── components/
│   │   └── MyComponent.jsx
│   └── utils/
│       └── helpers.js

Vite React App:

my-vite-app/
├── package.json
├── package-lock.json
├── node_modules/
├── public/
├── src/
│   ├── main.jsx
│   ├── App.jsx
│   └── components/
│       └── MyComponent.jsx
└── vite.config.js

---

Working Code Examples

Complete React 18 Setup:

// src/index.js
import React from 'react';
import { createRoot } from 'react-dom/client';
import './index.css';
import App from './App';

// Get the root container
const container = document.getElementById('root');

// Create root instance
const root = createRoot(container);

// Render the app
root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

App Component:

// src/App.jsx
import React, { useState } from 'react';
import './App.css';

function App() {
  const [count, setCount] = useState(0);

  return (
    <div className="App">
      <header className="App-header">
        <p>Count: {count}</p>
        <button onClick={() => setCount(count + 1)}>
          Increment
        </button>
      </header>
    </div>
  );
}

export default App;

---

Best Practices for React 18 Migration

1. Use Automatic Batching

React 18 automatically batches state updates for better performance:

// React 18 - Automatic batching
function Component() {
  const [count, setCount] = useState(0);
  const [flag, setFlag] = useState(false);

  function handleClick() {
    // Both updates are batched automatically
    setCount(c => c + 1);
    setFlag(f => !f);
  }

  return (
    <div>
      <p>Count: {count}</p>
      <p>Flag: {flag}</p>
      <button onClick={handleClick}>Update Both</button>
    </div>
  );
}

2. Handle Strict Mode Changes

React 18’s Strict Mode double-invokes effects in development:

// Proper cleanup for Strict Mode
function Component() {
  useEffect(() => {
    const timer = setInterval(() => {
      console.log('Timer tick');
    }, 1000);

    // Cleanup function is essential
    return () => {
      clearInterval(timer);
    };
  }, []);

  return <div>Component with cleanup</div>;
}

3. Use Suspense for Data Fetching

// With React 18 Suspense
function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <DataComponent />
    </Suspense>
  );
}

---

Debugging Steps

Step 1: Check React Version

npm list react react-dom

Step 2: Verify Import Statement

// ❌ Wrong
import ReactDOM from 'react-dom';

// ✅ Correct for React 18
import { createRoot } from 'react-dom/client';

Step 3: Clear Cache and Reinstall

rm -rf node_modules package-lock.json
npm install

Step 4: Check for Multiple React Versions

npm ls react --depth=10

---

Common Migration Issues and Solutions

Issue 1: Third-Party Library Compatibility

Some libraries may still use the old API. Check for updates or use compatibility layers.

Issue 2: Testing Library Updates

Update testing libraries to React 18 compatible versions:

npm install --save-dev @testing-library/react@latest

Issue 3: Custom Render Functions

If you have custom render functions, update them:

// Old custom render
function customRender(ui, options) {
  return ReactDOM.render(ui, document.createElement('div'));
}

// New custom render
function customRender(ui, options) {
  const container = document.createElement('div');
  const root = createRoot(container);
  root.render(ui);
  return { container, root };
}

---

Performance Considerations

1. Concurrent Features

React 18’s concurrent rendering can improve performance:

• Automatic batching
• Suspense for data fetching
• Transition updates

2. Memory Management

The new root API provides better memory management:

// Proper cleanup
const root = createRoot(container);
root.render(<App />);
// When unmounting
root.unmount();

---

Security Considerations

1. Input Validation

Always validate inputs regardless of React version:

function SafeComponent({ userInput }) {
  // Validate and sanitize inputs
  const sanitizedInput = sanitize(userInput);
  return <div>{sanitizedInput}</div>;
}

2. Dependency Updates

Keep React dependencies updated for security patches:

npm audit
npm audit fix

---

Testing the Migration

1. Development Server

npm start
# Should run without ReactDOM.render errors

2. Production Build

npm run build
# Should complete successfully

3. Unit Tests

npm test
# Should pass with updated React 18 APIs

---

Alternative Solutions

1. Temporary Compatibility Layer

If immediate migration isn’t possible:

// Temporary compatibility
import { createRoot } from 'react-dom/client';
import ReactDOM from 'react-dom';

const render = createRoot ? 
  (element, container) => {
    const root = createRoot(container);
    root.render(element);
  } : 
  ReactDOM.render;

export { render };

2. Downgrade (Not Recommended)

# Only as a temporary measure
npm install react@^17 react-dom@^17

---

Migration Checklist

• Update React and React DOM to v18+
• Replace ReactDOM.render with createRoot().render()
• Update import statements to use react-dom/client
• Update testing libraries
• Test all components for compatibility
• Verify production builds work correctly
• Update documentation and team members

---

Conclusion

The ‘ReactDOM.render is not a function’ error is a natural part of upgrading to React 18, which introduced significant improvements to React’s rendering system. By updating your application to use the new createRoot().render() API, you’ll gain access to React 18’s new features including automatic batching, concurrent rendering, and improved performance.

The migration process is straightforward: update your entry point files to use the new API, ensure you have React 18+ installed, and update any related dependencies. With these changes implemented, your React applications will be fully compatible with React 18 and ready to take advantage of its new capabilities.

Remember to test thoroughly after migration and update your team’s knowledge of the new React 18 patterns and best practices.