How to Fix "Hydration Mismatch" Errors in Next.js 15

The Problem

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

Why This Happens

Server-rendered HTML differs from client-rendered HTML. Common causes:

• Date/time formatting
• Browser-only APIs
• Random values
• User-specific data

Solution 1: Suppress Hydration Warning

// For timestamps
<time suppressHydrationWarning>
    {new Date().toLocaleString()}
</time>

Solution 2: useEffect for Client-Only Code

'use client';

import { useState, useEffect } from 'react';

export default function ClientTime() {
    const [time, setTime] = useState('');

    useEffect(() => {
        setTime(new Date().toLocaleString());
    }, []);

    if (!time) return null; // or skeleton

    return <span>{time}</span>;
}

Solution 3: Dynamic Import with ssr: false

import dynamic from 'next/dynamic';

const ClientOnlyChart = dynamic(
    () => import('./Chart'),
    { ssr: false }
);

Solution 4: Check for Browser Environment

'use client';

export default function BrowserOnly({ children }) {
    const [mounted, setMounted] = useState(false);

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

    if (!mounted) return null;

    return children;
}

Best Practices

• Keep Server Components as default
• Use 'use client' only when needed
• Move browser APIs to useEffect
• Use consistent date formatting