Server load functions (+page.server.js) in Svelte - Step-by-Step Execution

Concept Flow - Server load functions (+page.server.js)

Request from browser

↓

+page.server.js load function runs

↓

Fetch or compute data on server

↓

Return data as props

↓

Svelte page receives data

↓

Page renders with server data

↓

Send HTML to browser

When a page is requested, the +page.server.js load function runs on the server to get data, then passes it to the page for rendering.

Execution Sample

Svelte

export async function load() {
  const res = await fetch('https://api.example.com/data');
  const data = await res.json();
  return { data };
}

This load function fetches data from an API on the server and returns it as props for the page.

Execution Table

Step	Action	Evaluation	Result
1	Request received	Browser requests page	Trigger +page.server.js load
2	Run load function	Call load()	Start fetching data
3	Fetch data	await fetch('https://api.example.com/data')	Response object received
4	Parse JSON	await res.json()	Data object extracted
5	Return props	return { data }	Data passed to page
6	Page render	Page uses data props	HTML generated with data
7	Send response	Send HTML to browser	Page displayed to user

💡 Load function completes after returning props; page renders and response sent.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
res	undefined	Response object	Response object	Response object
data	undefined	undefined	Parsed JSON data	Parsed JSON data

Key Moments - 3 Insights

Why does the load function run on the server and not the browser?

Because +page.server.js load functions run only on the server to keep secrets safe and reduce client work, as shown in step 2 of the execution_table.

What happens if the fetch call fails inside the load function?

The load function will throw an error and the page won't render normally; error handling should be added to manage this, which is not shown in the simple example.

Why do we return data inside an object?

Returning { data } passes the data as props to the page component, enabling it to render with that data, as seen in step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what variable holds the parsed JSON data after step 4?

A res

B data

C props

D load

💡 Hint

Check the variable_tracker row for 'data' after step 4.

At which step does the page start rendering with the server data?

A Step 6

B Step 5

C Step 3

D Step 7

💡 Hint

Refer to the execution_table row describing 'Page render'.

If the fetch URL changes, which step in the execution_table will be directly affected?

A Step 5

B Step 2

C Step 3

D Step 6

💡 Hint

Look at the step where fetch is called.

Concept Snapshot

Server load functions (+page.server.js):
- Run only on server when page requested
- Fetch or compute data securely
- Return { ... } to pass data
- Page receives props and renders
- Keeps secrets safe and improves performance

Full Transcript

When a user requests a page, the +page.server.js load function runs on the server. It fetches or computes data needed for the page. This data is returned in an object. The page component receives these properties as props and renders HTML using the data. Finally, the server sends the rendered HTML to the browser. This process keeps sensitive operations on the server and improves page load speed.

Practice

1. What is the main purpose of a server load function in a +page.server.js file in SvelteKit?

easy

A. To fetch or prepare data on the server before the page loads

B. To handle client-side user interactions

C. To style the page components

D. To manage browser cookies on the client

Solution

1. Step 1: Understand server load function role

   Server load functions run on the server to get data before the page renders.

2. Step 2: Compare options to this role

   Only To fetch or prepare data on the server before the page loads describes fetching or preparing data on the server side before page load.

3. Final Answer:

   To fetch or prepare data on the server before the page loads -> Option A

4. Quick Check:

   Server load function = fetch data before page load [OK]

Hint: Server load functions run on server, not client [OK]

Common Mistakes:

• Confusing server load with client-side event handlers
• Thinking server load manages styles or cookies
• Assuming server load runs in the browser

2. Which of the following is the correct way to export a server load function in +page.server.js?

easy

A. export function load() { return { data: 'hello' }; }

B. export const load = () => { return { data: 'hello' }; }

C. export default function load() { return { data: 'hello' }; }

D. function load() { return { data: 'hello' }; }

Solution

1. Step 1: Recall SvelteKit server load export syntax

   Server load functions are exported as named exports, usually as a const arrow function.

2. Step 2: Check each option

   export const load = () => { return { data: 'hello' }; } uses export const load = () => { ... }, which is correct. export function load() { return { data: 'hello' }; } exports a function declaration but not as const, which is allowed but less common. export default function load() { return { data: 'hello' }; } uses default export which is incorrect. function load() { return { data: 'hello' }; } does not export at all.

3. Final Answer:

   export const load = () => { return { data: 'hello' }; } -> Option B

4. Quick Check:

   Named export const load = arrow function [OK]

Hint: Use named export const load = () => {} [OK]

Common Mistakes:

• Using default export instead of named export
• Not exporting the load function at all
• Using function declaration without export

3. Given this +page.server.js code, what will be the value of data.message in the page?

export const load = () => {
  return { message: 'Hello from server' };
};

medium

A. 'Hello from client'

B. undefined

C. An error occurs

D. 'Hello from server'

Solution

1. Step 1: Understand the load function return

   The load function returns an object with a property message set to 'Hello from server'.

2. Step 2: Recognize data passed to page

   The returned object properties become available as data in the page, so data.message equals 'Hello from server'.

3. Final Answer:

   'Hello from server' -> Option D

4. Quick Check:

   Returned object property = data property [OK]

Hint: Returned object keys become data properties [OK]

Common Mistakes:

• Expecting undefined if not wrapped in another object
• Confusing server message with client message
• Thinking load function causes error without async

4. What is wrong with this +page.server.js code?

export const load = async () => {
  const res = await fetch('/api/data');
  const data = await res.json();
  return data;
};

medium

A. The load function must not be async

B. Returning data directly instead of an object causes error

C. Fetching from '/api/data' on server causes an infinite loop

D. Missing export keyword

Solution

1. Step 1: Analyze fetch call in server load

   The load function fetches from '/api/data', which is likely a route handled by the same app.

2. Step 2: Understand server fetch behavior

   Fetching from an internal API route in server load can cause the server to call itself repeatedly, causing an infinite loop.

3. Final Answer:

   Fetching from '/api/data' on server causes an infinite loop -> Option C

4. Quick Check:

   Internal fetch in server load risks infinite loop [OK]

Hint: Avoid fetching internal routes directly in server load [OK]

Common Mistakes:

• Thinking async is not allowed in load
• Returning data directly is valid
• Forgetting to export load function

5. You want to load user data securely on the server and pass it to the page. Which approach correctly uses +page.server.js to fetch user info from a database and expose only the username?

hard

A. export const load = async () => {
const user = await db.getUser();
return { username: user.username };
};

B. export const load = async () => {
const user = await db.getUser();
return user;
};

C. export const load = () => {
fetch('/api/user').then(res => res.json());
return {};
};

D. export const load = async () => {
const user = await fetch('/api/user');
return { user };
};

Solution

1. Step 1: Identify secure data fetching on server

   export const load = async () => { const user = await db.getUser(); return { username: user.username }; }; fetches user data directly from the database on the server, which is secure.

2. Step 2: Check data exposure

   export const load = async () => { const user = await db.getUser(); return { username: user.username }; }; returns only the username property, exposing minimal info to the page.

3. Step 3: Evaluate other options

   export const load = async () => { const user = await db.getUser(); return user; }; returns full user object, exposing sensitive data. export const load = () => { fetch('/api/user').then(res => res.json()); return {}; }; uses client fetch incorrectly in server load. export const load = async () => { const user = await fetch('/api/user'); return { user }; }; fetches from API route which may cause loops or expose more data.

4. Final Answer:

   export const load = async () => { const user = await db.getUser(); return { username: user.username }; }; -> Option A

5. Quick Check:

   Server fetch minimal data, return only needed props [OK]

Hint: Fetch securely on server, return only needed data [OK]

Common Mistakes:

• Returning full user object exposing sensitive info
• Fetching from API route inside server load
• Using client fetch inside server load