What can run in server components in NextJS - Step-by-Step Execution

Concept Flow - What can run in server components

Start Server Component

↓

Run Server-only Code

↓

Fetch Data from DB or API

↓

Render JSX to HTML

↓

Send HTML to Client

↓

Client Hydrates Interactive Parts

↓

End

Server components run only on the server, fetching data and rendering HTML before sending it to the client.

Execution Sample

NextJS

export default async function Page() {
  const data = await fetch('https://api.example.com/data');
  const json = await data.json();
  return <div>{json.message}</div>;
}

This server component fetches data from an API and renders it as HTML on the server.

Execution Table

Step	Action	Code Executed	Result
1	Start Server Component	Page() function called	Component starts on server
2	Fetch Data	await fetch('https://api.example.com/data')	Data fetched from API
3	Parse JSON	await data.json()	JSON object received
4	Render JSX	return <div>{json.message}</div>	HTML string created with message
5	Send HTML	Server sends HTML to client	Client receives rendered HTML
6	Client Hydration	Client hydrates interactive parts	Page becomes interactive
7	End	Component rendering complete	No client-side code ran during render

💡 Server component finishes after rendering HTML and sending it to client; no client JS runs during server render.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
data	undefined	Response object from fetch	Response object	Response object
json	undefined	undefined	Parsed JSON object	Parsed JSON object

Key Moments - 3 Insights

Can server components run browser-only code like accessing window or document?

No, server components run only on the server where browser objects like window or document do not exist. See execution_table step 7 where no client JS runs during server render.

Can server components fetch data directly from a database?

Yes, server components can run server-only code like database queries or API calls as shown in execution_table steps 2 and 3.

Does server component code run on the client?

No, server component code runs only on the server. The client receives only the rendered HTML and hydrates interactive parts separately (execution_table step 6).

Visual Quiz - 3 Questions

Look at the execution table, at which step is data fetched from the API?

A Step 4

B Step 2

C Step 6

D Step 1

💡 Hint

Check the 'Action' column for 'Fetch Data' in the execution_table.

According to variable_tracker, what is the value of 'json' after step 3?

A Parsed JSON object

B Response object

C undefined

D null

💡 Hint

Look at the 'json' row and the 'After Step 3' column in variable_tracker.

If you try to use 'window' inside this server component, what happens?

A It works normally on the server

B It runs only after client hydration

C It causes an error because 'window' is not defined on the server

D It delays the server render

💡 Hint

Refer to key_moments about browser-only code in server components.

Concept Snapshot

Server Components in Next.js run only on the server.
They can fetch data from APIs or databases directly.
They render JSX to HTML before sending to the client.
No browser-only code (like window or document) can run here.
Client receives static HTML and hydrates interactive parts separately.

Full Transcript

Server components in Next.js run exclusively on the server. When a server component is called, it can perform server-only tasks such as fetching data from APIs or databases. The component then renders JSX into HTML on the server. This HTML is sent to the client, which then hydrates any interactive parts. Server components cannot use browser-specific objects like window or document because these do not exist on the server. The execution table shows the step-by-step process from starting the component, fetching data, rendering HTML, sending it to the client, and client hydration. The variable tracker shows how variables like the fetched data and parsed JSON change during execution. Key moments clarify common confusions about what can and cannot run in server components. The visual quiz tests understanding of these steps and concepts.