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.
- 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.
- 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.
- 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.
- 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.
- 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`.
