# Hydration Errors

Diagnose and fix React hydration mismatch errors.

## Error Signs

- "Hydration failed because the initial UI does not match"
- "Text content does not match server-rendered HTML"

## Debugging

In development, click the hydration error to see the server/client diff.

## Common Causes and Fixes

### Browser-only APIs

```tsx
// Bad: Causes mismatch - window doesn't exist on server
<div>{window.innerWidth}</div>;

// Good: Use client component with mounted check
('use client');
import { useState, useEffect } from 'react';

export function ClientOnly({ children }: { children: React.ReactNode }) {
  const [mounted, setMounted] = useState(false);
  useEffect(() => setMounted(true), []);
  return mounted ? children : null;
}
```

### Date/Time Rendering

Server and client may be in different timezones:

```tsx
// Bad: Causes mismatch
<span>{new Date().toLocaleString()}</span>;

// Good: Render on client only
('use client');
const [time, setTime] = useState<string>();
useEffect(() => setTime(new Date().toLocaleString()), []);
```

### Random Values or IDs

```tsx
// Bad: Random values differ between server and client
<div id={Math.random().toString()}>

// Good: Use useId hook
import { useId } from 'react'

function Input() {
  const id = useId()
  return <input id={id} />
}
```

### Invalid HTML Nesting

```tsx
// Bad: Invalid - div inside p
<p><div>Content</div></p>

// Bad: Invalid - p inside p
<p><p>Nested</p></p>

// Good: Valid nesting
<div><p>Content</p></div>
```

### Third-party Scripts

Scripts that modify DOM during hydration.

```tsx
// Good: Use next/script with afterInteractive
import Script from 'next/script';

export default function Page() {
  return <Script src="https://example.com/script.js" strategy="afterInteractive" />;
}
```
