Hydration is a key concept in modern web development frameworks like Next.js. If you’ve seen warnings like “Text content did not match” or experienced flickers between server-rendered and client-rendered pages, you’re likely running into hydration issues.

In this article, we’ll cover:

What hydration means in Next.js
Why hydration matters
Common hydration issues and fixes
Best practices to avoid hydration mismatches

What is Hydration in Next.js?

Hydration is the process where static HTML generated by the server is “enhanced” into a fully interactive React application on the client side. Next.js pre-renders HTML using SSR (Server-Side Rendering) or SSG (Static Site Generation), and then React takes over on the client to make it interactive.

graph LR
A[Server-rendered HTML] --> B[Client loads React]
B --> C[Hydration]
C --> D[Interactive App]

Why Does Hydration Matter?

Hydration enables the best of both worlds:

Fast initial load with server-rendered content (great for SEO)
Rich interactivity once React takes control

But if the HTML rendered on the server doesn’t match the one generated by React on the client, you’ll get hydration errors.

Common Hydration Issues

Mismatched content between server and client
Example: Using window, localStorage, or any browser-specific API during SSR.
Rendering dynamic data only available on the client
Example: Using timestamps or random IDs directly in component output.
Non-deterministic renders
Your component should render the same markup on server and client, unless wrapped in client-only logic.

How to Fix Hydration Issues in Next.js

1. Use useEffect for Client-Only Code

useEffect(() => {
  const localValue = localStorage.getItem("key");
  setState(localValue);
}, []);

Avoid running this logic during SSR.

2. Conditional Rendering with typeof window

if (typeof window === "undefined") {
  return null;
}

This helps skip rendering client-only code on the server.

3. Use Dynamic Imports with ssr: false

import dynamic from "next/dynamic";

const NoSSRComponent = dynamic(() => import("./MyComponent"), {
  ssr: false,
});

This loads the component only on the client.

Best Practices to Avoid Hydration Issues

Avoid using browser APIs during initial render.
Delay client-side logic using useEffect.
Keep server-rendered and client-rendered markup consistent.
Use next/dynamic for components that require the DOM.

Final Thoughts

Hydration is what makes React and Next.js powerful, blending server-rendered performance with dynamic user experiences. Understanding hydration helps you avoid common pitfalls, especially when building SEO-friendly, fast web apps.

Pro Tip: Use dev tools and console.warn to monitor hydration mismatches during development.

Understanding Hydration in Next.js: A Complete Guide

Prev:

02

ChatGPT vs Gemini vs DeepSeek: Which AI is Best for Web Development?