Fix: React Hydration Mismatch (Next.js 15)

Error: Hydration failed because the initial UI does not match what was rendered on the server.

What does this error mean?

This error occurs when the initial client-side render produces a different HTML structure than the server-side render. This is common when using dynamic data like dates, local storage, or window properties before the component mounts.

Common Causes

Accessing 'window' or 'localStorage' during initial render
Mismatched HTML nesting (e.g. <div> inside <p>)
Different results for Math.random() or new Date() between server and client

How to Fix It (1 Solution)

Use useEffect to wait for Client Mount

Only render dynamic parts of the UI after the component has mounted on the client.

Code

const [mount, setMount] = useState(false);
useEffect(() => setMount(true), []);
if (!mount) return null;

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