Hydration failed because the server-rendered HTML did not match the client....

When a web page is rendered on the server it loses its interactivity and event handles before getting to the client. 

Now react come into the picture. React adds the interactivity and event handlers that were lost when the HTML was server-side rendered.

Lets understand in some simple terms, This is a hydration error, and it happens when the server renders one version of your page, but the browser (client) renders something different.

It’s one of the most common issues in Next.js, especially if you're using dynamic content, third-party scripts, or certain hooks like useEffect.

What causes hydration errors in Next.js ?

Hydration is the process where react attaches interactivity to a pre-rendered HTML page.

 The server sends fully rendered HTML to the browser.

 React reuses this HTML but adds event listeners and dynamic content.

In short, If what the server rendered doesn’t match what the client renders, you get a hydration error.

Common Causes of Hydration Errors & Fixes

1. Mismatched Content Between Server & Client

 If the server and client render different content, React will throw a hydration error. Example:

export default function Home() {
  return <h1>{new Date().toLocaleTimeString()}</h1>;
}

The server renders a fixed timestamp when it builds the page.

The client re-renders with the current time, causing a mismatch.

import { useEffect, useState } from "react";

export default function Home() {
  const [time, setTime] = useState("");

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

  return <h1>{time}</h1>;
}

✔ Now, the server renders a placeholder (""), and the client updates the time after hydration.

2. Using window, document, or localStorage in SSR

 Server-side rendering (SSR) happens before the browser loads, so window, document, and localStorage don’t exist.

export default function Home() {
  return <h1>Screen Width: {window.innerWidth}</h1>;
}

❌ Error: window is not defined on the server

 Use useEffect() to access window on the client side.

import { useState, useEffect } from "react";

export default function Home() {
  const [width, setWidth] = useState(0);

  useEffect(() => {
    setWidth(window.innerWidth);
  }, []);

  return <h1>Screen Width: {width}</h1>;
}

✔ Now, the server renders an empty value (0), and the client updates it after hydration.

3. Using Random or Time-Based Values in Server Components

Values like Math.random() or Date.now() generate different results on the server and client, causing a mismatch.

export default function Home() {
  return <h1>Random Number: {Math.random()}</h1>;
}

 Generate the value only on the client side using useState & useEffect.

import { useState, useEffect } from "react";

export default function Home() {
  const [random, setRandom] = useState(null);

  useEffect(() => {
    setRandom(Math.random());
  }, []);

  return <h1>Random Number: {random}</h1>;
}

4. Conditional Rendering Based on Client Data

If a component renders differently on the server and client, React will throw a hydration error.

export default function Home() {
  return <h1>{navigator.userAgent}</h1>;
}

 navigator is only available on the client

Use useEffect() to update after hydration

import { useState, useEffect } from "react";

export default function Home() {
  const [userAgent, setUserAgent] = useState("");

  useEffect(() => {
    setUserAgent(navigator.userAgent);
  }, []);

  return <h1>{userAgent}</h1>;
}

5. Third-Party Scripts Running on the Server

Some third-party libraries (e.g., charts, ads, analytics) try to run on the server but rely on the browser.

import dynamic from "next/dynamic";

const Chart = dynamic(() => import("chart.js"), { ssr: false });

export default function Home() {
  return <Chart />;
}

✔ Now, the component only loads in the browser, preventing hydration issues.

Always use useEffect() for client-specific code (e.g., window, document, localStorage)

Use placeholders or default values for SSR-rendered content

Wrap third-party libraries with dynamic() and ssr: false

Avoid using random values (Math.random(), Date.now()) directly in server-rendered components

Use useState() for dynamic values and update them in useEffect()

How to Fix Hydration Errors in Next.js ? A Complete Guide

A slow website is so frustrating for end-users and too bad for SEO. If your Next.js site takes too much time to load, visitors might leave your website before they even see your content. Google also ranks faster websites higher, so speed is the most important thing.

 is built for performance, and with a few simple customization, you can make your site load faster and score well on 

Why speed matters for SEO and user experience ?

Common reasons why Next.js sites are slow.

Easy ways to optimize your site for faster loading times.

 End users leave your website if a page of it takes too much time to load. Lets see what the 

 is ? Its a metric that measures the percentages of website visitors who leave the website after viewing only one page. It shows how your website is connected with users.

 Nobody likes waiting for a page to load.

Google measures website performance using 

 How long it takes to load the biggest element on your page.

Any element with background image that loaded vis url() function

Block level elements that containing textor other inline level elements text.

The first frame of an animated image format, like any animated GIF

Anything that the outside of the viewport or any overflow as well is not considering while measuring the LCP. For example, If any image occupies the entire viewport its not considered for LCP.

.It sholuld be based on chrome user experience report (CrUX) data.

 How quickly your page responds to user actions.

Events like scrolling or zooming are excluded

now what is CrUX data ? Its a data from the actual users of chrome who are on your site. You need 

 How stable your page layout is while loading.

 duration while the most shifing occurs. The main reason behind this occurs is when you try to click somthing ona page that shifts and then ending up by clicking on somthing you dont want to.

Images without dimensions [ widht and height ]

Any ads, embeds, and iframes without dimensions

Web fonts that causes FOIT [Flash of Invisible Text] or FOUT [Flash of Unstyled Text] 

. It should be based on Chrome User Experience Report (CrUX) data.

Now, let’s look at how to improve page speed in Next.js ? 

Easy Ways to Optimize Page Speed in Next.js

1.  Optimize Images with Next.js <Image> Component

In our regular HTML, we have added any image like below snippet,

We have to manually optimize below things,

Image is responisive based on the screen sizes

Optimize image with any third party toll or library

As the image is about to enter on viewport lazy load them.

Large images may slow down websites. Next.js provides an optimized <Image> component that,

Lazy loads images (loads them only when needed)

Automatically converts them to modern formats like WebP

Resizes images for different screen sizes

import Image from "next/image";

<Image 
  src="/example.png" 
  width={400} 
  height={400} 
  alt="Example Image" 
  priority
/>

 for images above the fold (visible when the page first loads).

2.  Use Static Site Generation (SSG) for Faster Loading

If your page doesn’t change frequently then pre-render it at the build time using SSG. This makes it load quickly without waiting for a server response.

export async function getStaticProps() {
  const res = await fetch("https://api.example.com/data");
  const data = await res.json();

  return { props: { data } };
}

✔ It is best for blogs, landing pages and marketing webistes.

3. Enable Incremental Static Regeneration (ISR) for Updated Content

If your content changes frequently, but you still want the speed of SSG, use ISR. This updates static pages without rebuilding the entire website.

export async function getStaticProps() {
  const res = await fetch("https://api.example.com/data");
  const data = await res.json();

  return { props: { data }, revalidate: 60 }; // Updates every 60 sec
}

✔ It is perfect for news sites, blogs, and product pages.

Too much javascript may slow down a website. Next.js allows lazy loading, which means scripts load only when required.

import dynamic from 'next/dynamic';

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

Lazy load third-party scripts like Google Analytics:

import Script from "next/script";

<Script 
  src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXXX-X" 
  strategy="lazyOnload" 
/>

✔ It reduces unnecessary network requests.

module.exports = {
  purge: ['./pages/**/*.js', './components/**/*.js'],
};

Analyze your JavaScript bundle with Webpack Bundle Analyzer:

npm install --save-dev @next/bundle-analyzer

const withBundleAnalyzer = require('@next/bundle-analyzer')({
  enabled: process.env.ANALYZE === 'true',
});

module.exports = withBundleAnalyzer({});

How to Optimize or Improve Google Page Speed in Next.js?

 have become widely popular because they separate content management from frontend view.

This allows developers to use flexible, API-driven content delivery, that makes websites load faster and more scalable.

, is one of the best choices for integrating a headless CMS. 

, making it an perfect match for headless CMS platforms like Strapi, Contentful, Sanity, WordPress (headless mode).

What a headless CMS is and how it works ?

Why Next.js is a perfect fit for headless CMS integration ?

The benefits of using Next.js with a headless CMS.

A step-by-step guide to integrating a headless CMS with Next.js.

The best headless CMS options for Next.js.

Let’s get started by exploring what the headless cms is ?

 is a content management system that separates the information and presentation. Too much technical stuff right ? Lets understand in simple tone,

 is a tool or system that helps you create and manage content like images, text, videos,etc for your website, but it 

doesn’t decide how that content is shown

 where you store your books [here its content]. A regular CMS just like wordpress is like having the librarian not only store the books but also show them to you on a shelf.

A headless CMS is like a library where you store the books, but it leaves it up to you or a dedicated developer to decide where and how to display them.

it focuses on content storage and lets you deliver that content anywhere

Your content is created and stored in the headless CMS.

The CMS provides a REST or GraphQL API to fetch the respective content.

Next.js consumes the API and displays content dynamically.

Above decoupled architecture makes content management more flexible, scalable, and future-proof.

Why is Next.js Perfect for Headless CMS Integration ?

Next.js widely offers multiple rendering methods , API routes, and built-in optimizations that make it an perfect choice for working with headless CMS platforms.

Server-Side Rendering (SSR) for Real-Time Content

 Blogs, news websites, and dynamic pages.

Next.js supports SSR, meaning content can be fetched from the headless CMS at request time. This is useful for sites that require real-time updates without needing a full rebuild.

export async function getServerSideProps() {
  const res = await fetch("https://cms-api.com/posts");
  const posts = await res.json();
  
  return { props: { posts } };
}

Static Site Generation (SSG) for SEO & Performance

 Marketing sites, landing pages, and product pages.

SSG [Static Site Generation] allows Next.js to pre-render pages at build time, reducing server load and improving SEO and performance.

export async function getStaticProps() {
  const res = await fetch("https://cms-api.com/posts");
  const posts = await res.json();
  
  return { props: { posts } };
}

Next.js allows you to create API endpoints within your project, enabling custom CMS logic like webhook handling or content transformations.

export default function handler(req, res) {
  res.status(200).json({ message: "Hello from Next.js API!" });
}

Incremental Static Regeneration (ISR) for Hybrid Content Updates

 E-commerce, blogs, or pages needing periodic updates.

With ISR, you can update static pages without rebuilding the entire site, making Next.js extremely efficient for dynamic content.

export async function getStaticProps() {
  const res = await fetch("https://cms-api.com/posts");
  const posts = await res.json();
  
  return { props: { posts }, revalidate: 60 }; // Updates every 60 seconds
}

Next.js automatically optimizes images fetched from a headless CMS, improving performance and Google PageSpeed scores.

import Image from "next/image";

<Image src="https://cms-api.com/images/post.jpg" width={500} height={300} alt="Blog Image" />;

Step-by-Step Guide: Integrating a Headless CMS with Next.js

Please follow these below steps to integrate a headless CMS with Next.js,

Below is the popular headless cms options among which you can make a perfect decision for your website,

 – Real-time collaborative content editing.

getServerSideProps [for real-time updates]

Map through the fetched data to display it dynamically.

export default function Blog({ posts }) {
  return (
    <div>
      {posts.map((post) => (
        <h2 key={post.id}>{post.title}</h2>
      ))}
    </div>
  );
}

Step 4: Deploy to Vercel for Best Performance

Next.js is optimized for Vercel, which ensures seamless deployment and global CDN distribution.

Best For : Best for Enterprises & Cloud Hosting

Best For : Best for Real-Time Collaboration

Why Next.js Is Ideal for Headless CMS Integration ?

, formerly a Typekit, offers a wide range of high-quality fonts that can give your 

Unlike the Google Fonts, which can be self-hosted via next/font/google, Adobe Fonts requires a slightly different approach. Let's see what that is…

This blog will cover below important points,

What is  best way to integrate Adobe Fonts in Next.js ?

How to use local hosting for better performance Optimizing Adobe Fonts for better Core Web Vitals ?

Before you begin, you’ll need an Adobe account. If you don’t have one yet, just sign up on Adobe Fonts.

Browse through the font library and pick the ones you love and perfect for your website. There's a huge selection, so you're sure to find something perfect for your site.

Once you've found a font you want to use, click on it and then hit Add to Web Project.

When you add a font to your project, Adobe will ask you to create a Web Project.

Just give your project a name (something like “My Awesome Website Fonts” or something you like), and pick the font styles you want (like bold or italic). This helps you only load the ones you need, making your site faster.

Once your project is ready, Adobe will generate a special embed code for you.

Please Copy the code — it’s a simple <link> tag that you’ll use in your website’s HTML.

Paste the embed code into the <head> section of your page, like this:

This links your website to the fonts you just picked.

Now that the fonts are connected to your website, it’s time to apply them! In your CSS file, you can use the font like this:

body {
    font-family: "Your Chosen Font", sans-serif;
}

Save your changes, refresh your website, and voilà! Your new fonts should be live on your site.

Lets see how this magic works on next.js as well.

Method 1: Using <link> in _document.js which is Recommended for SEO & Performance.

Since Next.js optimizes font loading, adding Adobe Fonts to _document.js ensures styles are applied efficiently.

=> Open pages/_document.js and add the Adobe Fonts link inside <Head>

import { Html, Head, Main, NextScript } from "next/document";

export default function Document() {
  return (
    <Html>
      <Head>
        <link rel="stylesheet" href="https://use.typekit.net/your-project-id.css" />
      </Head>
      <body>
        <Main />
        <NextScript />
      </body>
    </Html>
  );
}

This method loads fonts early and ensures no layout shift (CLS). and Fonts will be cached, improving performance.

Method 2: Adding Adobe Fonts in _app.js or a Global CSS File

Alternatively, you can import the Adobe Fonts CSS in _app.js or your global styles file.

import "../styles/globals.css"; 

export default function App({ Component, pageProps }) {
  return <Component {...pageProps} />;
}

Then, add the Adobe Fonts import inside globals.css:

@import url("https://use.typekit.net/your-project-id.css");

body {
  font-family: "your-font-name", sans-serif;
}

Option B: Load Fonts Dynamically in a Component

If you want to load fonts only when a specific component is used, you can do:

import { useEffect } from "react";

export default function MyComponent() {
  useEffect(() => {
    const link = document.createElement("link");
    link.href = "https://use.typekit.net/your-project-id.css";
    link.rel = "stylesheet";
    document.head.appendChild(link);
  }, []);

  return <p style={{ fontFamily: "your-font-name" }}>Hello, Adobe Fonts!</p>;
}

Use this method if you need fonts in specific pages only.

Once the fonts are loaded, you can use them in CSS files or styled-components.

h1 {
  font-family: "your-font-name", serif;
}

For Styled Components (If Using Emotion or Styled-Components):

import styled from "styled-components";

const Title = styled.h1`
  font-family: "your-font-name", serif;
`;

export default function Home() {
  return <Title>Hello with Adobe Fonts!</Title>;
}

Optimizing Adobe Fonts for Performance & SEO

To ensure fast loading and good Core Web Vitals, follow these tips:

 If a font is used in the hero section, preload it by adding this inside _document.js:

Too many fonts increase load time. Try to use only necessary weights & styles.

 by default, which can cause FOIT (Flash of Invisible Text). To fix this, use:

@font-face {
  font-family: "your-font-name";
  src: url("https://use.typekit.net/your-project-id.css");
  font-display: swap;
}

Alternative: Self-Hosting Adobe Fonts in Next.js

If you want even faster performance, you can self-host Adobe Fonts instead of loading them from Adobe's CDN.

Download the Font Files from Adobe Fonts (if allowed).

@font-face {
  font-family: "CustomAdobeFont";
  src: url("/fonts/custom-adobe-font.woff2") format("woff2"),
       url("/fonts/custom-adobe-font.woff") format("woff");
  font-weight: normal;
  font-style: normal;
}

body {
  font-family: "CustomAdobeFont", sans-serif;
}

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