Fixing React/Next.js Build Error: "Document is not defined"

Understanding the Error

This error commonly occurs when deploying React or Next.js applications to platforms like Vercel or in build time. It's puzzling because it usually doesn't appear during local development (localhost:3000).

Why it happens:

1. During build time, the bundled code undergoes server-side checks.

2. The error occurs when parts of the code require a browser environment to execute.

3. The server can't provide these browser-specific objects, leading to the error.

   As developers, we need to ensure that browser-dependent code only runs after the website mounts.

The Role of DOM in React

React uses the DOM as a bridge between HTML and JavaScript, allowing easy manipulation of HTML content when the website mounts. This manipulation can't happen on the server, so components requiring DOM access to manipulate are called "client components."

To verify this, check the Network tab in your browser's dev tools. You'll see that static content is visible, while client-side content is not.

Understanding the Concept of Server-Side Rendering (SSR) vs. Client-Side Rendering (CSR)

Understanding the difference between SSR and CSR is crucial before fixing this error:

• SSR: The initial content is rendered on the server, improving SEO and initial load time.

• CSR: The content is rendered in the browser, allowing for dynamic updates.

Next.js uses a hybrid approach, which can sometimes lead to this error when not handled correctly.

Solutions

Let's explore several methods to resolve this issue:

Method 1: Using useEffect

This method uses React's useEffect hook to ensure the component only renders client-side:

import { useState, useEffect } from 'react';

function YourComponent() {
    const [isMounted, setIsMounted] = useState(false);

    useEffect(() => {
        setIsMounted(true);
    }, []);

    if (!isMounted) return null;

    return <div>Your component content</div>;
}

Method 2: React Lazy Loading

This method is preferred as it's more performant than using useEffect. React.lazy() optimizes performance by delaying component rendering until the browser needed.

import { lazy, Suspense } from 'react';

const LazyComponent = lazy(() => import('./YourComponent'));

function App() {
    return (
        <Suspense fallback={<div>Loading...</div>}>
            <LazyComponent />
        </Suspense>
    );
}

This approach:

• Reduces initial JS bundle size

• Makes the component browser and user-dependent

• Uses Suspense to show a loading state, maintaining UI consistency

Method 3: Next.js Dynamic Imports

Next.js provides a built-in solution with dynamic imports:

import dynamic from 'next/dynamic';

const DynamicComponent = dynamic(() => import('./YourComponent'), {
    ssr: false,
});

function App() {
    return <DynamicComponent />;
}

This method is an optimized version of React.lazy() particularly useful in Next.js applications as it's optimized for the framework.

Method 4: Conditional Rendering with typeof window

A simple check can prevent the component from rendering server-side:

function YourComponent() {
    if (typeof window === 'undefined') return null;

    return <div>Your component content</div>;
}

Method 5: Custom Hook for Browser Detection

Create a reusable hook to check for browser environment:

import { useState, useEffect } from 'react';

function useIsBrowser() {
    const [isBrowser, setIsBrowser] = useState(false);

    useEffect(() => {
        setIsBrowser(true);
    }, []);

    return isBrowser;
}

// Usage
function YourComponent() {
    const isBrowser = useIsBrowser();

    if (!isBrowser) return null;

    return <div>Your component content</div>;
}

By implementing either of these methods, you can effectively resolve the "Document is not defined" error and ensure your React/Next.js application builds and deploys successfully.