Understanding and Fixing Hydration Errors in Next.js

Hydration errors are a common pitfall for developers new to Next.js, especially when dealing with client components that rely on browser APIs like localStorage. In this article, we’ll explore what hydration errors are, how they occur, and how to fix them effectively, with intuitive code examples to illustrate the concepts.

What Are Hydration Errors?

Hydration errors occur when there’s a mismatch between the HTML rendered on the server and the HTML that React generates on the client after hydration. This can lead to inconsistencies in the UI, resulting in unexpected behavior.

Example Scenario: Using localStorage

Let's say we have a simple client component that checks whether an item is favorited using localStorage. Here's a basic example:

"use-client"
import { useEffect, useState } from 'react';

const FavoriteButton = () => {
    const [isFavorited, setIsFavorited] = useState(false);

    // Check localStorage on the client side
    useEffect(() => {
        if (typeof window !== 'undefined') {
            const favorite = localStorage.getItem('favorite');
            setIsFavorited(favorite === 'true');
        }
    }, []);

    return (
        <button>
            {isFavorited ? 'Unfavorite' : 'Favorite'}
        </button>
    );
};

export default FavoriteButton;

How It Works

1. Server-Side Rendering: During the initial server render, the component executes, but since localStorage is not accessible, isFavorited remains false. Thus, the server sends the following HTML to the client:

    <button>Favorite</button>
   

2. Client-Side Hydration: When the component mounts on the client side, useEffect runs, and it checks localStorage. If the value is true, it updates the state to reflect this, leading to a re-render with:

    <button>Unfavorite</button>
   

This mismatch between the server-rendered HTML and the client-rendered HTML results in a hydration error.

Fixing Hydration Errors

Method 1: Suppress Hydration Warnings

If you are okay with knowing that there might be a hydration mismatch but want to suppress the warning, you can use the suppressHydrationWarning attribute. Here’s how to implement it:

import { useEffect, useState } from 'react';

const FavoriteButton = () => {
    const [isFavorited, setIsFavorited] = useState(false);

    useEffect(() => {
        if (typeof window !== 'undefined') {
            const favorite = localStorage.getItem('favorite');
            setIsFavorited(favorite === 'true');
        }
    }, []);

    return (
        <button suppressHydrationWarning>
            {isFavorited ? 'Unfavorite' : 'Favorite'}
        </button>
    );
};

export default FavoriteButton;

Method 2: Correct HTML Structure

Hydration errors can also occur due to incorrect HTML structure, such as placing a <div> inside a <p> tag. If the HTML structure is incorrect, browsers may try to correct it when rendering on the client side, which can result in a mismatch between the server-rendered HTML and the client-rendered HTML. Ensure your HTML follows proper semantic structure. For example:

<!-- Incorrect -->
<p>
    <div>This is a div inside a paragraph.</div>
</p>

<!-- Correct -->
<div>
    <p>This is a paragraph.</p>
</div>

Conclusion

Hydration errors can be a frustrating issue when developing with Next.js, especially when using client components that interact with browser-specific APIs. By understanding how these errors arise and applying the methods outlined in this article, you can ensure smoother hydration and a better user experience in your applications. Always validate your initial state, maintain correct HTML structure, and feel free to suppress warnings when necessary to keep your development process streamlined.