Page options (SSR, CSR, prerender) in Svelte - Step-by-Step Execution

Concept Flow - Page options (SSR, CSR, prerender)

Page Request

↓

SSR

↓

Render HTML

↓

Send to Browser

↓

User Sees Page

When a page is requested, SvelteKit checks its setup to decide how to render: Server-Side Rendering (SSR), Client-Side Rendering (CSR), or Prerendering. Then it sends the HTML to the browser accordingly.

Execution Sample

Svelte

export const ssr = true;
export const csr = false;
export const prerender = false;

This code sets the page to render only on the server (SSR), disables client-side rendering (CSR), and disables prerendering.

Execution Table

Step	Page Option Checked	Value	Action Taken	Resulting Output
1	ssr	true	Render page HTML on server	HTML generated on server
2	csr	false	No client-side JS hydration	Static HTML sent
3	prerender	false	No prerendering at build time	Page rendered on demand
4	Request complete	-	-	Browser receives server-rendered HTML

💡 All options checked; page rendered on server without client JS or prerendering.

Variable Tracker

Variable	Initial	After Step 1	After Step 2	After Step 3	Final
ssr	undefined	true	true	true	true
csr	undefined	undefined	false	false	false
prerender	undefined	undefined	undefined	false	false

Key Moments - 3 Insights

Why does setting csr to false mean no client-side JavaScript runs?

Because csr=false disables client-side hydration, so the browser only gets static HTML without interactive JS. See execution_table step 2.

What happens if prerender is true but ssr is false?

Prerendering generates static HTML at build time, so the page is served as static files without server rendering on each request.

Can a page have both SSR and CSR enabled?

Yes, ssr=true and csr=true means the page is server-rendered initially and then hydrated on the client for interactivity.

Visual Quiz - 3 Questions

Look at the execution_table, what is the action taken when ssr is true?

A Render page HTML on server

B Disable server rendering

C Generate static files at build time

D Run client-side JavaScript only

💡 Hint

Check execution_table row 1 under 'Action Taken'

At which step does the system decide not to run client-side JavaScript?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table step 2 and the 'csr' variable effect

If prerender is set to true, how would the output change?

A Page HTML is generated at build time and served as static files

B Page is rendered only on client side

C Page is never rendered

D Page is rendered on server on every request

💡 Hint

Refer to key_moments explanation about prerender true

Concept Snapshot

Page options in SvelteKit control how pages render:
- ssr=true: Render on server on each request
- csr=true: Enable client-side JavaScript hydration
- prerender=true: Generate static HTML at build time
Combine these to optimize performance and interactivity.

Full Transcript

When a user requests a page in SvelteKit, the system checks the page options: ssr, csr, and prerender. If ssr is true, the page HTML is generated on the server for each request. If csr is true, client-side JavaScript runs to hydrate the page for interactivity. If prerender is true, the page is built as static HTML at build time and served as static files. These options control when and where the page renders, balancing speed and interactivity. For example, setting ssr true and csr false means the page is server-rendered but static on the client. Setting prerender true means the page is pre-built and served without server rendering on each request.

Practice

1. What does SSR (Server-Side Rendering) do in a Svelte page?

easy

A. It sends fully rendered HTML from the server to the browser.

B. It renders the page only after JavaScript loads in the browser.

C. It builds static pages ahead of time during the build process.

D. It disables JavaScript on the page for faster loading.

Solution

1. Step 1: Understand SSR concept

   SSR means the server prepares the full HTML before sending it to the browser.

2. Step 2: Compare with other options

   CSR waits for JavaScript to render, prerender builds static pages ahead, and disabling JavaScript is unrelated.

3. Final Answer:

   It sends fully rendered HTML from the server to the browser. -> Option A

4. Quick Check:

   SSR = server sends ready HTML [OK]

Hint: SSR means server sends ready HTML, not browser rendering [OK]

Common Mistakes:

• Confusing SSR with CSR
• Thinking prerender is the same as SSR
• Believing SSR disables JavaScript

2. Which of the following is the correct way to disable SSR for a Svelte page?

easy

A. export const csr = true;

B. export const prerender = false;

C. export const ssr = false;

D. export const ssr = true;

Solution

1. Step 1: Recall syntax to disable SSR

   In Svelte, setting export const ssr = false; disables server-side rendering.

2. Step 2: Check other options

   prerender controls static generation, csr is not a valid option, and ssr = true enables SSR.

3. Final Answer:

   export const ssr = false; -> Option C

4. Quick Check:

   Disable SSR with ssr = false [OK]

Hint: Use 'export const ssr = false;' to disable SSR [OK]

Common Mistakes:

• Using 'prerender = false' to disable SSR
• Trying to set 'csr = true' which is invalid
• Confusing enabling and disabling SSR

3. Given this Svelte page code:

export const prerender = true;
export const ssr = false;

What will happen when this page is built and served?

medium

A. The page is statically generated at build time and rendered only on the client.

B. The page is server-rendered on each request and not prerendered.

C. The page is neither prerendered nor server-rendered, causing an error.

D. The page is prerendered and server-rendered on every request.

Solution

1. Step 1: Analyze prerender and ssr flags

   Setting prerender = true means the page is built as a static HTML file ahead of time. Setting ssr = false disables server-side rendering on requests.

2. Step 2: Understand combined effect

   The page is generated once at build time (prerendered) and then served as static HTML. Since SSR is off, no server rendering happens on requests; rendering happens on the client.

3. Final Answer:

   The page is statically generated at build time and rendered only on the client. -> Option A

4. Quick Check:

   prerender true + ssr false = static build + client render [OK]

Hint: prerender true + ssr false means static build + client render [OK]

Common Mistakes:

• Thinking SSR still runs on requests
• Assuming no rendering happens causing error
• Confusing prerender with SSR

4. You want a Svelte page to be fully static and load instantly without server rendering. You write:

export const ssr = false;
export const prerender = false;

What is the problem with this setup?

medium

A. The page will be server-rendered but not prerendered, causing slow loads.

B. The page will not prerender and also disables SSR, so it won't generate HTML before client loads.

C. The page will cause a syntax error because both options cannot be false.

D. The page will prerender but still use SSR, causing duplicate rendering.

Solution

1. Step 1: Understand the effect of both flags false

   Setting ssr = false disables server-side rendering, and prerender = false disables static generation.

2. Step 2: Identify the consequence

   Without SSR or prerender, the page has no prebuilt HTML and relies on client-side rendering only, causing a blank page until JavaScript runs.

3. Final Answer:

   The page will not prerender and also disables SSR, so it won't generate HTML before client loads. -> Option B

4. Quick Check:

   ssr false + prerender false = no prebuilt HTML [OK]

Hint: Both ssr and prerender false means no prebuilt HTML [OK]

Common Mistakes:

• Assuming prerender happens automatically
• Thinking this causes syntax error
• Believing SSR still runs when false

5. You want a Svelte page that:
- Is prerendered as static HTML for fast loading
- Uses SSR only if the user is logged in
- Otherwise, renders on the client

Which setup best achieves this behavior?

hard

A. export const prerender = false;
export const ssr = false;
// Fully client-side rendering only

B. export const prerender = false;
export const ssr = true;
// Server renders always regardless of login

C. export const prerender = true;
export const ssr = false;
// Use client-side code to check login and fetch data

D. export const prerender = true;
export const ssr = true;
// Use server hooks to check login and render accordingly

Solution

1. Step 1: Understand prerender and SSR roles

   Prerender true builds static HTML at build time. SSR true enables server rendering on requests.

2. Step 2: Match requirements

   To prerender for fast loading and also use SSR conditionally (like checking login), you need both prerender and SSR enabled. Server hooks can check login and decide rendering.

3. Step 3: Evaluate other options

   export const prerender = true; export const ssr = false; // Use client-side code to check login and fetch data disables SSR, so no server rendering on login. export const prerender = false; export const ssr = true; // Server renders always regardless of login disables prerender, losing static HTML. export const prerender = false; export const ssr = false; // Fully client-side rendering only disables both, no server or static rendering.

4. Final Answer:

   export const prerender = true; export const ssr = true; // Use server hooks to check login and render accordingly -> Option D

5. Quick Check:

   Prerender + SSR true for static + conditional server render [OK]

Hint: Enable both prerender and SSR for static plus conditional server render [OK]

Common Mistakes:

• Disabling SSR when conditional server render is needed
• Disabling prerender losing static HTML benefits
• Assuming client code can replace server login checks