How To Fix Hydration Failed Because The Initial UI Does Not Match

Learn why hydration errors occur in React and Next.js, what causes mismatched UI between server and client, and how to fix them. This guide covers browser-only APIs, random values, and safe rendering techniques to help keep your app stable and predictable.

Have you ever refreshed your Next.js or React app only to see the error: “hydration failed because the initial UI does not match what was rendered on the server”? This means the HTML from the server and the initial UI from the client did not align, causing mismatched components and warnings.

In this blog, we’ll explain why hydration errors happen, what rendering logic causes them, and the practical steps to fix them in React and Next.js.

Understanding Hydration in Server-Side Rendering

When using server-side rendering (SSR), the server renders HTML and sends it as a response to the client. On the client side, React attaches event handlers and reuses the same content. If the initial client-side render does not match the expected server HTML, hydration errors occur.

What Hydration Really Means

Hydration is not about rendering twice, but rather attaching event handlers to pre-rendered HTML. The idea is that the server sends a ready HTML response, and React only connects interactive logic. When differences exist between server-rendered HTML and the client-rendered HTML, hydration mismatch warnings start appearing.

Why This Problem Happens

The problem occurred because certain rendering logic modifying conditions on the client side inevitably differ from those on the server. Browser-only APIs, such as window or document, random text content, or incorrect nesting inside HTML tags, can lead to a mismatch. React expects the same content server-side and client-side for hydration to succeed.

Common Reasons for Hydration Errors

When developers encounter the error "hydration failed” because the initial UI does not match what was rendered on the server, it often ties back to rendering logic differences.

Browser-Only APIs Causing Issues

Since the server does not have access to browser APIs, any rendering logic that depends on typeof window or navigator will break. This means browser APIs like local storage or screen width must only run on the client side.

Conditional Rendering Issues

Conditional rendering sometimes creates different HTML structures. For instance, if you intentionally render a component differently based on window size, the server-rendered HTML and initial UI will not match.

Incorrect Nesting and HTML Tags

Hydration mismatches can also occur due to incorrect nesting, such as nested li tags or mismatched HTML tags. Even minor variations in text content may lead to different content between the server and the client.

Random Values or Dates

If your rendering logic includes random values, timers, or new Date, you will get hydration mismatches. The server-rendered HTML will never match what was rendered during the initial client-side render.

Fixing Hydration Failed Errors in Next.js

Solving hydration mismatch requires handling both server-side and client-side rendering logic carefully.

Using Dynamic Imports

Next.js provides a given method called import dynamic to delay rendering of specific components until the client side. For instance:

Copy
1import dynamic from 'next/dynamic';
2
3const NoSSRComponent = dynamic(() => import('../components/ClientComponent'), {
4  ssr: false
5});
6
7export default function App() {
8  return <NoSSRComponent />;
9}
10

Here, export default function app prevents the component from being server rendered, avoiding hydration mismatch.

Guarding with typeof Window

Use typeof window checks to protect browser-only APIs. This prevents rendering logic from breaking server-rendered HTML.

Copy
1export default function SampleComponent() {
2  if (typeof window === 'undefined') return null;
3  return <div>Client-only logic</div>;
4}
5

Using useEffect Hook

The useEffect hook runs only on the client side after hydration, making it safe for attaching event handlers or handling browser APIs.

Copy
1export default function Component() {
2  React.useEffect(() => {
3    console.log(window.innerWidth);
4  }, []);
5  return <div>Safe rendering</div>;
6}
7

Avoiding Random Values in Initial Render

Never allow rendering logic to generate random or different HTML content between the server and the client. Instead, use stable defaults and update values with useState hook or useEffect hook.

If you want to see how other developers tackled the same issue, check out this GitHub discussion on hydration errors . It highlights real-world cases and different approaches shared by the Next.js community.

Handling Hydration Mismatches in React Tree

Matching Server Rendered HTML with Client Side

React expects that the same content server-side will appear in the client render. If text content differs or HTML content mismatches, React hydration fails. Developers must focus on stable rendering logic, modifying only on the client side with proper hooks.

Attaching Event Handlers Safely

Always attach event handlers during or after hydration. Do not let specific components bind events conditionally during server-side rendering.

Avoiding Performance Issues

Performance issues arise when developers rerun large parts of the React tree solely to address hydration issues. Instead, focus on resolving mismatches at the component level.

Best Practices to Prevent Hydration Errors

Preventing hydration mismatches requires careful practices during SSR and client-side rendering.

Keep Rendering Logic Deterministic

Rendering logic should not inevitably differ between server and client. Avoid different HTML elements or random outputs in the first render.

Use Pre-Rendered Content Consistently

Make sure the pre-rendered HTML response matches the initial client-side render. This avoids hydration mismatch warning errors.

Split Client-Only Components

Mark specific components as client-only using import dynamic or guard them with typeof window. Use export default dynamic with SSR disabled when required.

Correctly Nest HTML Tags

Avoid incorrect nesting or invalid HTML tags. A small mistake, such as differences in div classnames or incorrect nesting, can trigger hydration errors.

Example: Fixing a Hydration Error

Copy
1import dynamic from 'next/dynamic';
2
3const SafeClientComponent = dynamic(() => import('../components/SafeComponent'), { ssr: false });
4
5export default function App() {
6  return (
7    <div>
8      <SafeClientComponent />
9    </div>
10  );
11}
12

This method prevents hydration mismatch when the client's only logic means browser APIs are necessary.

Want to build apps without worrying about hydration errors or rendering mismatches? Start creating reliable, production-ready apps today with Rocket.new.

Build Your App Now!

Key Takeaway: Preventing Hydration Failure Errors

When you face hydration failure because the initial UI does not match what was rendered on the server, the core idea is to keep the rendering logic predictable between the server-side and client-side.

By separating browser APIs, using import dynamic, and relying on hooks like useEffect for client updates, hydration mismatches can be avoided. Following these methods keeps your React tree consistent, reduces hydration mismatches, and delivers stable server-side rendering results.