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Hydration Mismatch Error - Server and Client Render Different Content

Next.js throws a hydration mismatch error. The HTML from server doesn't match what React renders on client. Content flickers or changes when page loads.

Error: 'Warning: Did not expect server HTML to contain a

in
'

Error Messages You Might See

Warning: Did not expect server HTML to contain Hydration failed because the initial HTML differs from the render Text content does not match server-rendered HTML

Common Causes

  1. Using client-only features (browser APIs) during server render
  2. Rendering different content based on window/document being defined
  3. Timestamps or random data rendered server-side changing on client
  4. Conditional rendering based on isClient flag without useEffect
  5. CSS-in-JS not being applied on server during build

How to Fix It

Move browser code to useEffect: useEffect(() => { setMount(true); }, [])

Use dynamic imports for browser-only components: dynamic(() => import('@/components/ClientComponent'), { ssr: false })

Don't use Date.now(), Math.random() during render - do it in useEffect

Check if typeof window === 'undefined' before using browser APIs

Real developers can help you.

Anthony Akpan Anthony Akpan Developer with 8 years of experience building softwares fro startups legrab legrab I'll fill this later Simon A. Simon A. I'm a backend developer building APIs, emulators, and interactive game systems. Professionally, I've developed Java/Spring reporting solutions, managed relational and NoSQL databases, and implemented CI/CD workflows. Yovel Cohen Yovel Cohen I got a lot of experience in building Long-horizon AI Agents in production, Backend apps that scale to millions of users and frontend knowledge as well. Basel Issmail Basel Issmail ’m a Senior Full-Stack Developer and Tech Lead with experience designing and building scalable web platforms. I work across the full development lifecycle, from translating business requirements into technical architecture to delivering reliable production systems. My work focuses on modern web technologies, including TypeScript, Angular, Node.js, and cloud-based architectures. I enjoy solving complex technical problems and helping teams turn product ideas and prototypes into working platforms that can grow and scale. In addition to development, I often collaborate closely with product managers, business analysts, designers, and QA teams to ensure that solutions align with both technical and business goals. I enjoy working with startups and product teams where I can contribute both as a hands-on engineer and as a technical partner in designing and delivering impactful software. MFox MFox Full-stack professional senior engineer (15+years). Extensive experience in software development, qa, and IP networking. Costea Adrian Costea Adrian Embedded Engineer specilizing in perception systems. Latest project was a adas camera calibration system. zipking zipking I am a technologist and product builder dedicated to creating high-impact solutions at the intersection of AI and specialized markets. Currently, I am focused on PropScan (EstateGuard), an AI-driven SaaS platform tailored for the Japanese real estate industry, and exploring the potential of Archify. As an INFJ-T, I approach development with a "systems-thinking" mindset—balancing technical precision with a deep understanding of user needs. I particularly enjoy the challenge of architecting Vertical AI SaaS and optimizing Small Language Models (SLMs) to solve specific, real-world business problems. Whether I'm in a CTO-level leadership role or hands-on with the code, I thrive on building tools that turn complex data into actionable value. Taufan Taufan I’m a product-focused engineer and tech leader who builds scalable systems and turns ideas into production-ready platforms. Over the past years, I’ve worked across startups and fast-moving teams, leading backend architecture, improving system reliability, and shipping products used by thousands of users. My strength is not just writing code — but connecting product vision, technical execution, and business impact. Luca Liberati Luca Liberati I work on monoliths and microservices, backends and frontends, manage K8s clusters and love to design apps architecture

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Frequently Asked Questions

When does hydration happen?

After Next.js sends HTML from server, React takes over in browser (hydration) and attaches event handlers

Why does my timestamp differ between server and client?

Server renders at build time or request time. Client renders milliseconds later. Timestamps won't match. Move to useEffect

How do I debug hydration issues?

Enable verbose logging: add suppressHydrationWarning as temporary fix, then find actual mismatch

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