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Error: Prop className did not match in Next.js: Causes and How to Fix

February 10, 2025

Discover common causes of the Next.js "Prop className did not match" error and learn effective solutions with our comprehensive guide.

What is Error: Prop className did not match in Next.js

 

Error: Prop className Did Not Match in Next.js

 

  • The message "Error: Prop className did not match" in Next.js occurs during the hydration process when React reconciles server-rendered HTML with client-side rendered HTML. This error implies that the `className` prop for a particular component or element is inconsistent between the server rendering and the client rendering.

    •  

  • Hydration is an important step in using a server-side rendered (SSR) app with React or Next.js because it ensures that users can interact with the site immediately. However, differences in how the server and client render things, such as CSS class names, can lead to issues, with this error message being one of them.

 

Understanding the Hydration Process

 

  • When your Next.js application renders a page on the server-side, it sends HTML to the client that represents the application. This HTML includes attributes like `class` for elements, based on the render output on the server.

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  • Once the HTML arrives at the client side, React needs to "hydrate" it. This means React takes over the HTML and binds it to the React components. This process allows the React app to become interactive.

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  • If the client's initial render produces HTML with different `className` properties than what was on the server, a mismatch error appears, indicating that the HTML has inconsistencies.

 

Implications of the Error

 

  • This error is typically non-fatal in that it does not break the interaction or functionality of the application usually, but it indicates potential issues in your app's rendering logic.

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  • An inconsistent `className` can be indicative of a broader issue of differing app states between the server and client, which could break user expectations in complex applications.

 

Typical Scenarios Where This Occurs

 

  • Conditional Rendering: When conditions that apply during the server-side rendering are different from those at client-side rendering, resulting in different classes being applied.

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  • Dynamic Class Names: When using libraries or methods to dynamically generate CSS class names, such as CSS Modules or styled-components, changes during hydration can lead to a mismatch.

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  • User-Agent-Based Styles: If you have code that changes the styling of components based on the user's browser or device type and this code is only executed client-side.

 

Example of Error Manifestation

 

const MyComponent = () => {
  const [isClient, setIsClient] = useState(false);

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

  return (
    <div className={isClient ? 'client-class' : 'server-class'}>
      Hello, world!
    </div>
  );
};

 

  • In the above example, the `className` will differ between server and client because `isClient` starts as `false` during server rendering and becomes `true` after the component mounts, causing the className to change from `server-class` to `client-class`.

 

What Causes Error: Prop className did not match in Next.js

 

Understanding the Error Message: "Prop className did not match"

 

  • This error occurs predominantly in Next.js applications. It is often associated with React's Strict Mode or when there's a discrepancy between server-rendered and client-rendered HTML.

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  • The message indicates a mismatch between the server-rendered HTML and the client-rendered HTML specifically concerning the `className` prop of an element. This discrepancy can cause hydration issues.

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  • The problem arises from different styles or conditions applied on the server and the client. For instance, when CSS styling uses dynamic classNames based on JavaScript that executes differently on the client compared to the server.

 

Potential Causes of the Error

 

  • Conditional Rendering Based on Browser-Specific APIs: When rendering conditionally using browser-specific properties such as `window` or `navigator`, the client and server can generate different outputs since these properties are undefined on the server.
const isClient = typeof window !== 'undefined';

const MyComponent = () => {
  return (
    <div className={isClient ? 'client-class' : 'server-class'}>
      Content
    </div>
  );
};

 

  • Inconsistent CSS-In-JS: Libraries that rely on runtime evaluation, like styled-components or emotion, might apply different styles when CSS-in-JS does not match during hydration.

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  • Environment-Based Styling: Using environment variables to change classNames between production and development environments without consistent settings can result in this error.

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  • Asynchronous Data Fetching: Misalignment between data received by server-render and what is re-fetched or modified on the client-side can change the expected classNames after a component is hydrated.

 

Example of Conditional Rendering Based on Data

 

const MyComponent = ({ user }) => {
  const statusClass = user.isLoggedIn ? 'logged-in' : 'logged-out';
  return (
    <div className={statusClass}>
      User Content
    </div>
  );
};

 

  • If `user` changes between server render and client render, className discrepancies will occur.

 

Conclusion

 

  • The "Prop className did not match" error in Next.js applications often arises from inconsistent server and client rendering caused by reliance on browser-specific states, dynamic styling conditions, or asynchronous operation differences.

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  • Identifying and resolving these can improve compatibility and ensure seamless hydration between server and client.

 

How to Fix Error: Prop className did not match in Next.js

 

Check Your CSS-in-JS Solution

 

  • If using styled-components, ensure you have a consistent version server- and client-side. Consider upgrading to the latest version if discrepancies remain.

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  • For emotion, ensure the correct use of their cache and ssr setups as per their documentation so that styles are generated correctly both on the server and client.

 

Use Next.js Head for Global Styles

 

  • Make sure to use the `