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How to Fix the Hydration Mismatch Error in Next.js App Router

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A common error in Next.js App Router occurs when the HTML rendered on the server does not match what React generates during client-side hydration, causing warnings and unpredictable behavior. The mismatch is typically triggered by direct use of browser APIs like window or localStorage, conditional logic outside of hooks, or invalid nested HTML elements. Developers can resolve this by using useEffect to defer client-only logic, applying suppressHydrationWarning on affected elements, or disabling SSR entirely via Next.js dynamic imports with ssr: false. Invalid HTML nesting, such as block elements inside paragraphs, is another frequent culprit that must be corrected to comply with web standards. Testing in incognito mode can help determine whether browser extensions like Grammarly or Dark Reader are injecting DOM changes before hydration.

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How to Fix the Hydration Mismatch Error in Next.js App Router · ShortSingh