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Next.js

Fix Next.js document is not defined Error

By Filip on 04/21/2024

Learn how to resolve the "document is not defined" error in Next.js, a common issue encountered when working with client-side code that tries to access the document object before it's available.

Fix Next.js document is not defined Error

Table of Contents

Introduction

New to Next.js and encountering the frustrating "document is not defined" error? Don't worry, you're not alone. This guide will unravel the mystery behind this error, common in server-side rendering (SSR), and equip you with step-by-step solutions to overcome it. We'll explore the root cause and delve into various techniques like conditional rendering, React hooks, dynamic imports, and custom document configurations to ensure your Next.js applications run smoothly.

Step-by-Step Guide

The "document is not defined" error is a common hurdle when working with Next.js, especially for those new to server-side rendering (SSR). Let's break down the cause and explore solutions step-by-step:

Understanding the Root Cause:

  • Server-Side Rendering: Next.js renders pages on both the server and the client. The document object, however, is only available on the client-side (in the browser). When your code tries to access document during server-side rendering, it throws this error.

Solutions:

Here are several approaches to resolve the issue, depending on your specific scenario:

1. Conditional Rendering with process.browser:

  • Explanation: This method checks if the code is running on the client-side before accessing document.
  • Steps:
    1. Import process.browser: 
      import { process } from 'next/dist/server/web/sandbox';
    2. Wrap your code: 
      if (process.browser) {
    // Your code accessing 'document' goes here
    const element = document.getElementById('myElement');
    // ...
}

2. Using useEffect Hook (for React Components):

  • Explanation: This approach ensures your code runs only after the component mounts in the browser.
  • Steps:
    1. Import useEffect: 
      import { useEffect } from 'react';
    2. Use useEffect to access document: 
      useEffect(() => {
    // Your code accessing 'document' goes here
    const element = document.getElementById('myElement');
    // ...
}, []); // Empty dependency array to run only once

3. Dynamic Imports with next/dynamic (for Third-Party Libraries):

  • Explanation: This method dynamically loads components that require document only on the client-side.
  • Steps:
    1. Import dynamic: 
      import dynamic from 'next/dynamic';
    2. Dynamically import the component: 
      const MyComponent = dynamic(() => import('./MyComponent'), { ssr: false });

4. Custom _document.js (for Advanced Use Cases):

  • Explanation: This allows you to customize the entire HTML document structure and inject scripts or styles.
  • Steps:
    1. Create a _document.js file in your pages directory.
    2. Extend the Document class and override the getInitialProps method to control initial rendering.
    3. Access document within the render method.

Additional Tips:

  • Identify the source: Pinpoint where the error occurs to choose the most suitable solution.
  • Consider alternatives: Explore alternative libraries or approaches that don't rely on document if possible.
  • Test thoroughly: Ensure your solution works as expected on both the server and client sides.

By understanding the cause and applying these solutions, you can effectively overcome the "document is not defined" error and build robust Next.js applications.

Code Example

This code provides solutions for the "document is not defined" error in Next.js, which occurs when trying to access the document object on the server-side. The examples demonstrate four methods:

  1. Conditional Rendering: Using process.browser to execute code that accesses 'document' only on the client-side.
  2. useEffect Hook: Employing the useEffect hook to run code after the component mounts in the browser, ensuring 'document' is available.
  3. Dynamic Imports: Utilizing next/dynamic to dynamically import components and prevent server-side rendering, thus avoiding the error.
  4. Custom _document.js: Creating a custom _document.js file to access and modify the document object for specific use cases.

Following your provided explanations, here are the JavaScript code examples for each solution:

1. Conditional Rendering with process.browser:

import { process } from 'next/dist/server/web/sandbox';

function MyComponent() {
  if (process.browser) {
    // Client-side code accessing 'document'
    const element = document.getElementById('myElement');
    console.log(element.textContent);
  }

  return <div id="myElement">Hello, world!</div>;
}

export default MyComponent;

2. Using useEffect Hook:

import { useEffect } from 'react';

function MyComponent() {
  useEffect(() => {
    // Code accessing 'document' after component mounts
    const element = document.getElementById('myElement');
    console.log(element.textContent);
  }, []); // Empty array ensures it runs only once

  return <div id="myElement">Hello, world!</div>;
}

export default MyComponent;

3. Dynamic Imports with next/dynamic:

import dynamic from 'next/dynamic';

const MyComponent = dynamic(
  () => import('./MyComponent'), // Replace with your component path
  { ssr: false } // This ensures client-side rendering
);

function MyPage() {
  return (
    <div>
      <h1>My Page</h1>
      <MyComponent />
    </div>
  );
}

export default MyPage;

4. Custom _document.js (Example):

// pages/_document.js

import Document, { Html, Head, Main, NextScript } from 'next/document';

class MyDocument extends Document {
  render() {
    return (
      <Html>
        <Head>
          {/* Your custom head content */}
        </Head>
        <body>
          <Main />
          <NextScript />
          {/* Access 'document' here if needed */}
          <script>console.log(document.title);</script>
        </body>
      </Html>
    );
  }
}

export default MyDocument;

Remember:

  • Choose the solution that best fits your specific use case.
  • For more complex scenarios, consider combining these approaches.
  • Always test your code thoroughly to ensure it works as expected on both the server and client sides.

Additional Notes

While the provided solutions effectively address the "document is not defined" error in most cases, there are additional nuances and troubleshooting tips to consider:

1. Third-Party Libraries and SSR Compatibility:

  • Challenge: Some third-party libraries might not be designed for SSR and may directly access document or window objects, leading to the error.
  • Solution:
    • Check documentation: Review the library's documentation for SSR compatibility or specific instructions for Next.js usage.
    • Dynamic imports: Use next/dynamic to load the library only on the client-side if SSR support is lacking.
    • Alternatives: Explore alternative libraries that offer SSR support or similar functionality.

2. CSS-in-JS Libraries and Server-Side Styles:

  • Challenge: Certain CSS-in-JS libraries might require access to document during server-side rendering for style injection.
  • Solution:
    • Library-specific solutions: Refer to the library's documentation for recommended approaches to handle SSR.
    • Server-side rendering APIs: Utilize Next.js APIs like getServerSideProps or getInitialProps to inject critical styles on the server.

3. Debugging and Error Isolation:

  • Challenge: Identifying the exact source of the error can be tricky, especially in larger codebases.
  • Solution:
    • Console logs: Use console.log(process.browser) to pinpoint whether the code is running on the server or client.
    • Breakpoints and debugging tools: Leverage browser developer tools or debugging extensions to step through your code and inspect variables.
    • Code isolation: Try to isolate the problematic code snippet to narrow down the issue.

4. Custom Document and Potential Conflicts:

  • Challenge: Customizing _document.js can introduce conflicts if not handled carefully.
  • Solution:
    • Structure and order: Ensure proper structure and order of elements within the custom document.
    • Script loading: Pay attention to script loading strategies and potential conflicts with third-party scripts.
    • Testing: Thoroughly test your custom document implementation to avoid unexpected behavior.

Summary

Solution Explanation When to Use
Conditional Rendering with process.browser Checks if code is running on client-side before accessing document. Simple cases where you need to access document in specific parts of your code.
useEffect Hook Runs code only after component mounts in the browser. When you need to access document within a React component.
Dynamic Imports with next/dynamic Dynamically loads components requiring document on the client-side. When using third-party libraries that rely on document.
Custom _document.js Customizes HTML structure and injects scripts/styles. Advanced use cases requiring full control over the document structure.

Conclusion

Throughout this guide, we've demystified the "document is not defined" error in Next.js, a common challenge arising from the framework's server-side rendering capabilities. We've explored the root cause ā€“ the absence of the document object during server-side rendering ā€“ and equipped you with a toolkit of solutions:

  • Conditional Rendering: Employ process.browser to selectively execute code that interacts with document only on the client-side.
  • useEffect Hook: Leverage this React hook to ensure your code runs after the component mounts in the browser, guaranteeing the availability of document.
  • Dynamic Imports: Utilize next/dynamic to dynamically load components that require document solely on the client-side, preventing server-side conflicts.
  • Custom _document.js: For advanced scenarios, create a custom _document.js file to tailor the HTML structure and inject scripts or styles as needed.

Remember, the choice of solution hinges on your specific use case and the context of the error. Consider the complexity of your application, the libraries you're using, and the level of control you require over the document structure.

Beyond the Basics:

We've also delved into advanced considerations and troubleshooting tips to tackle more intricate situations:

  • Third-Party Library Compatibility: Be mindful of SSR compatibility when using third-party libraries, opting for dynamic imports or alternative solutions if necessary.
  • CSS-in-JS and Server-Side Styles: Explore library-specific recommendations or Next.js APIs to handle server-side style injection effectively.
  • Debugging Techniques: Utilize console logs, breakpoints, and code isolation to pinpoint the source of the error and streamline your debugging process.
  • Custom Document Best Practices: Ensure proper structure, script loading strategies, and thorough testing when customizing _document.js.

Empowered to Build Robust Next.js Applications:

Armed with this knowledge, you're well-equipped to conquer the "document is not defined" error and build robust, performant Next.js applications that seamlessly navigate the complexities of server-side rendering. Keep exploring, experimenting, and pushing the boundaries of what's possible with this powerful framework!

References

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