The error message "hydration failed because the initial UI does not match what was rendered on the server" occurs when the React client-side tree tries to attach to a different HTML structure than what was pre-rendered on the server. This mismatch disrupts the hydration process, causing React to fail to properly attach event handlers and make the app interactive

Common Causes of Hydration Mismatch

• Incorrect HTML nesting : For example, a <div> inside a <p>, or interactive elements nested improperly (like <a> inside another <a>) can cause differences between server and client rendering

• Conditional rendering differences : Using browser-only APIs (like window or localStorage) or checks like typeof window !== 'undefined' during rendering can cause the server and client to render different content

• Time-dependent or random values : Using new Date() or Math.random() directly in render causes server and client to output different content because the values differ at render time

• Third-party libraries or component libraries that render differently on server and client

• Browser extensions or CDN modifications that alter the HTML between server and client

How to Fix Hydration Errors

• Ensure consistent rendering on server and client : Components should render the same HTML initially on both sides. Avoid using browser-only APIs or time-dependent values directly in the render phase

• UseuseEffect for client-only logic: Move client-specific rendering or state updates into useEffect, which only runs on the client after hydration, preventing mismatches

    jsx
    
    import { useState, useEffect } from 'react';
    
    export default function App() {
      const [isClient, setIsClient] = useState(false);
      useEffect(() => {
        setIsClient(true);
      }, []);
      return <h1>{isClient ? 'Client content' : 'Server content'}</h1>;
    }

• Disable SSR for problematic components : In Next.js, use dynamic imports with { ssr: false } to prevent server-side rendering of components that cause mismatches

• Fix HTML nesting issues : Correct invalid HTML structures that browsers may tolerate but servers do not

• UsesuppressHydrationWarning for minor unavoidable differences: This React prop silences warnings when content differs but should be used sparingly

• Conditionally render time-dependent content : For example, render a placeholder on the server and the actual date/time on the client after hydration

Summary

Hydration errors arise because the server-rendered HTML and the client- rendered React tree do not match exactly. The main solution is to ensure that the initial render output is identical on both server and client by avoiding browser-only APIs, fixing HTML structure, and deferring client-specific rendering to useEffect or disabling SSR for certain components