What is Hydration in Next.js?

Hydration is the process that Next.js uses to connect the JavaScript code running on the client with the HTML that was previously rendered from the server. This process allows React to take over the UI without having to re-render the entire page.

However, if not managed correctly, hydration can cause errors and impact performance.

1. Hydration Mismatch

This error occurs when the HTML rendered from the server is different from the HTML that React renders on the client. Common causes include using time-sensitive data like random numbers, dates, or asynchronous APIs during server-side rendering.

export default function Page() {
  return <p>{Math.random()}</p>;
}

How to fix:

• Avoid using time-sensitive data during server-side rendering.
• If dynamic data is needed, update it after the component mounts.

2. Accessing Browser APIs on the Server

The server does not have access to window, document, or localStorage, so if the code tries to access these during rendering, an error will occur.

import { useState, useEffect } from "react";

export default function Page() {
  const [width, setWidth] = useState(0);
  
  useEffect(() => {
    setWidth(window.innerWidth);
  }, []);

  return <p>Width: {width}</p>;
}

How to fix:

• Only access browser APIs after the component has mounted.
• Use hooks like useEffect to handle logic related to the browser.

3. Forgetting to add 'use client' when needed

By default, components in the app/ directory of Next.js 13+ are Server Components. If a component needs to use useState, useEffect, or event listeners without 'use client', it will cause an error.

'use client';
import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}

How to fix:

• Add 'use client' at the beginning of the file if the component needs state or user events.
• Clearly distinguish between Server Components and Client Components.

4. Too many Client Components causing loss of Server Components benefits

Some developers apply 'use client' across the entire application, causing Next.js to lose the benefits of Server Components, which increases the JavaScript bundle size.

How to fix:

• Only use 'use client' when necessary.
• Keep static components like layout and navigation as Server Components.

5. How to avoid Hydration errors

• Avoid time-sensitive data during server-side rendering.
• Do not use browser APIs in Server Components.
• Only add 'use client' for components that need interactivity.
• Use a good balance between Server Components and Client Components.

Hydration is an important part of Next.js, but if not properly understood, it can lead to errors and performance issues. Optimize your code to avoid these problems and take full advantage of Next.js!