Examining the mechanisms of server-side rendering and hydration in Gatsby and Next

**Understanding Server-Side Rendering with React**

**The Issue: React Component Rendering in the Wrong Spot**

Have you ever encountered a situation where your React component rendered in the wrong spot in production, even though it worked perfectly fine in development? This unexpected behavior can be frustrating and confusing. Upon investigating the issue using the devtools’ Elements tab, you discovered that the culprit behind this rendering problem was a fundamental misunderstanding about how React works in a server-side-rendering context. This misunderstanding is shared by many React developers and can have serious implications.

**The Problem of Client-Side vs Server-Side Rendering**

To understand this problem better, let’s explore the difference between client-side rendering and server-side rendering when using frameworks like Gatsby and Next.js compared to traditional client-side apps built with React.

In client-side rendering, which is the default behavior when building applications with React, the rendering occurs in the user’s browser. When the browser receives the initial HTML document, it appears empty, with only a couple of JS scripts. The browser downloads and parses these scripts, and then React comes into play, building the page and injecting DOM nodes to create the desired UI. However, this process can take some time, leaving the user staring at a blank screen until the rendering is complete.

On the other hand, server-side rendering (SSR) aims to improve the user experience by rendering the HTML on the server-side, allowing the user to see a fully-formed HTML document while the browser handles the remaining tasks. The server transforms React components into HTML and sends it to the user, reducing the blank screen time. However, this render still occurs on-demand, meaning that the user still has to wait for it to happen.

**Compile-Time Rendering for Performance Optimization**

To optimize performance, developers realized that many parts of websites and apps are static and can be pre-built at compile-time. Tools like Gatsby and Next.js generate HTML files for each route and pre-render them during the build process. This approach, known as static site generation (SSG) or server-side generation (SSG), produces an HTML document for every page, blog post, or store item, ready to be served immediately. This way, React apps can load quickly, similar to vanilla HTML sites.

**Hydration: Making Client-Side JS Compatible with Pre-rendered HTML**

While the server-side generated HTML allows the user to see the initial content faster, modern web apps still require client-side JavaScript for interactivity. The client-side JavaScript includes the same React code used during compile-time rendering. It runs on the user’s device, compares the pre-rendered HTML with the current UI state, and then applies any necessary updates. This process is known as hydration, where React assumes that the DOM won’t change significantly, and it simply tries to adopt the existing DOM structure.

**The Problem with Misunderstanding Server-Side Rendering**

Returning to the initial issue of the React component rendering in the wrong spot, we can now identify the problem. In an attempt to handle the user’s login state, the code snippet provided checks if the rendering occurs on the server by verifying the existence of the window object. This approach seems reasonable as the initial build happens in a server runtime. However, it violates the rules of React’s hydration process.

When React hydrates, it expects the DOM structure to match the pre-rendered HTML. By rendering different content depending on whether it’s in a server-side render or not, the code tries to bypass this crucial rule. While React may occasionally handle such situations, it’s not guaranteed to work correctly. The hydration process is designed for speed, not for fixing mismatches.

**The Solution: A Different Approach to Handling Server-Side Rendering**

To solve this rendering issue and properly handle server-side rendering, a different approach is needed. Rather than rendering different content based on server-side or client-side rendering, we should focus on creating a consistent initial UI state for all users.

One solution is to default to the “logged out” state for all users in the pre-rendered HTML. This way, there won’t be any content flickering or incorrect UI representations during the initial loading phase. Once the client-side JavaScript executes and hydrates the app, it can then update the UI based on the user’s authentication state.

This approach ensures that all users see a consistent initial UI, and any changes or updates happen seamlessly during the hydration process. By following this approach, you can avoid the rendering issues caused by misinterpreting server-side rendering and adhere to React’s hydration rules.


Understanding server-side rendering with React is essential to avoid unexpected rendering issues. By comprehending the differences between client-side and server-side rendering and following proper hydration practices, you can ensure a smooth and consistent user experience. Remember that misusing server-side rendering can lead to compatibility problems and potential errors in your React applications. Always strive for clarity and consistency in your code to avoid these pitfalls.

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