How to Fix React 19 Hydration Mismatch Errors

The Hydration Headache in 2026

Hydration mismatches happen when the HTML generated by the server (Next.js) doesn't perfectly match the structure React tries to build on the client.

### Why React 19 is Different

React 19 introduces stricter check routines. Common causes include using Math.random() or new Date() inside a component, which will always differ between the server execution and the client execution.

### The Fix

The most robust way to fix this is to wrap the client-only logic in a useEffect.

``tsx

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

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

return isClient ? : ;

``

This ensures that the server-rendered output matches the initial client-rendered output, and the "real" content is only injected once the client is ready.