If you're a junior developer who has worked with Next.js, you've probably encountered this question in technical interviews:
"Is use client exclusively for Client-Side Rendering (CSR)?"
If you can confidently answer "No, components with use client can also utilize SSR (Server-Side Rendering)!", you'll leave a strong impression on your interviewer.
In this article, we'll focus on SSR and Server Components to clearly understand what use client really means and what the Hydration process entails. We'll also explore how all React components were client components before Server Components came along, and examine the performance benefits of Server Components to help you prepare thoroughly for Next.js interviews.
Understanding SSR, CSR, and Hydration
Traditionally, React applications often operated with CSR (Client-Side Rendering). The browser receives basic HTML and JS bundles from the server, but the initial screen only contains an empty <div id="root"></div>, leaving users staring at a blank screen until JS loads and rendering completes. This delays initial loading times and negatively impacts SEO.
One solution to this problem is SSR (Server-Side Rendering). With SSR, React components are rendered to HTML on the server before being sent to the client, allowing users to see actual content instead of a blank screen during initial load. However, this HTML is still in a "dry (Markup-only)" state without interactivity, and only becomes fully interactive after the browser loads JavaScript and React completes its hydration process by registering event handlers.
What is Hydration?
Hydration is the process where static HTML rendered on the server is matched with React's virtual DOM on the client side, and event handlers are attached to create an interactive UI. In other words, after receiving "structure-only" HTML through SSR, once JavaScript loads and hydrate() executes, event and state management logic becomes active on top of this HTML structure.
[Server: SSR] --(HTML)--> [Browser: Initial Display]
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(JS Load, hydrate())
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[Browser: Event Handlers Attached, VDOM Sync]
Now users can see content immediately upon page load, with interactivity becoming available as soon as JS is ready.
Distinguishing Server Components from Client Components
Server Components, introduced after React 18, presented a new paradigm of "components that only run on the server." In Next.js, all components are Server Components by default, and only components that need to run on the client explicitly declare use client.
Server Components are delivered to the client as HTML snippets and are excluded from hydration. This means the HTML returned by Server Components is simply inserted into the browser "as is," without event handlers or state being injected on the client side. As a result, Server Components consume minimal JavaScript bundle size, significantly helping with performance optimization.
On the other hand, components declaring use client behave like traditional React components. They generate initial HTML through SSR, then go through the hydration process to activate interactivity. Therefore, use client components can utilize SSR and support the hydration process that combines SSR + CSR.
Before Server Components: All Components Were Client Components
Before Server Components existed, all React components had to go through hydration on the client side. Even with SSR, all components ultimately needed to have their logic re-injected with JavaScript on the client. This led to large bundle sizes and longer initial loading times in large applications.
After the introduction of Server Components, we can now handle "static parts that only need to be displayed" as Server Components without any declaration, while parts requiring interactivity can be marked with use client. This clear separation enables more efficient bundle optimization and better performance.
How Server Components Work: Slot Insertion and HTML Combination
Server Components render HTML snippets on the server and provide them to the client, while React's render tree leaves a "hole" for these snippets. The final HTML is created by inserting these snippets into the corresponding holes. Here's a simple ASCII diagram illustrating this process:
[Server: RSC(Render)] --> [HTML snippet]
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[React Render Tree]: "hole" creation
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Combined HTML --> Client transmission
(Server Components excluded from hydration)
In this process, Server Components do not include JavaScript bundles. As a result, they cannot register event handlers or state, but this is actually beneficial for parts that only require static HTML. The reduced JavaScript bundle size leads to faster initial screen display.
Acing Your Interview: Confidently Answering "Is 'use client' Only for CSR?"
For example, you might encounter this question in an interview:
Interviewer: "Do components with use client only support CSR?"
Model Answer:
"No, in Next.js, all components are Server Components by default, and only components that need to run on the client explicitly declare use client. These client components generate initial HTML through SSR and then go through the hydration process to activate interactivity on the client side. Therefore, use client components can also utilize SSR. On the other hand, Server Components are excluded from hydration and only provide static HTML, reducing JavaScript bundle size and improving performance."
If you can clearly explain this, the interviewer will recognize that you have a solid grasp of Server Components, SSR, and hydration concepts.
Server-Side Rendering and Server Components Are Not Mutually Exclusive
Finally, it's essential to note that Server Components do not replace SSR. SSR focuses on displaying content to users immediately during initial load, while Server Components provide an additional layer for optimizing bundle size and data fetching. These two technologies complement each other, and using them together enables you to create a better user experience and improve performance.
Bonus: What Is 'use server'?
'use server' is a different concept from Server Components, used to define Server Actions. Server Actions allow clients to directly call server functions, making them useful for handling server-side tasks like form submissions or data updates.
Recap
- All Next.js components are Server Components by default
- Client components require explicit
use clientdeclaration - Client components can utilize SSR: server-side HTML rendering followed by hydration
- Server Components provide static HTML without hydration, reducing JavaScript bundle size and improving performance
- 'use server' is a separate feature for defining Server Actions
- Server Components complement SSR, not replace it: together, they provide optimal user experience and performance
Armed with this knowledge, confidently answer interview questions. Emphasize that "use client components also support SSR" and explain how Server Components lead to bundle optimization and performance improvements. Highlighting that all components are Server Components by default and only client components need the 'use client' directive will demonstrate your deep understanding of Next.js architecture.
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