React4 min read2026-03-01

Next.js Hydration Error in App Router

Fix hydration failed and server/client mismatch errors specific to Next.js Server Components and Client Components.

Error Code / Stack Trace

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

Problem Overview

Next.js App Router renders HTML on the server, but client-side JavaScript produces different DOM nodes during initial load.

Why Does This Happen?

Accessing window, document, or localStorage inside Client Components without a mounted check.
Mismatch in date/time formatting between server locale and client user locale.
HTML syntax violations such as nesting <div> inside <p> or <table> without <tbody>.

Step-by-Step Solution

Step 1: Use dynamic imports with ssr: false for client-only widgets

For widgets that strictly rely on client browser state (e.g. geolocation, canvas), disable server rendering.

tsx

import dynamic from 'next/dynamic';

const ClientMap = dynamic(() => import('@/components/Map'), {
  ssr: false,
  loading: () => <p>Loading map...</p>,
});

Common Mistakes to Avoid

•Using 'use client' and assuming it completely disables server rendering. Client components in Next.js are still pre-rendered on the server!

Prevention & Best Practices

Format dates to a standardized UTC format or use suppressHydrationWarning.

Frequently Asked Questions

Does 'use client' make a component run exclusively on the client?

No, 'use client' merely marks the boundary where client interactivity is enabled; Next.js still pre-renders its initial HTML on the server.