Server load functions (+page.server.js) in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a +page.server.js file in SvelteKit?

It contains server-side load functions that run only on the server to fetch or prepare data before the page renders. This keeps sensitive logic and data secure.

beginner

How does a server load function differ from a client load function in SvelteKit?

Server load functions run only on the server and can access secrets or databases safely. Client load functions run in the browser and cannot access server-only resources.

beginner

What is the typical export name for a server load function inside +page.server.js?

The function is exported as load. This is the standard name SvelteKit looks for to run server-side data loading.

intermediate

Can you access URL parameters inside a server load function in +page.server.js? How?

Yes, the server load function receives an argument with a params object containing URL parameters. You can use params.id or similar to get values.

beginner

Why should you avoid putting sensitive API keys or secrets in client-side code and instead use +page.server.js?

Because client-side code is visible to users and can be inspected, exposing secrets. Server load functions run only on the server, keeping secrets hidden and secure.

Where does the load function inside +page.server.js run?

A Only on the server

B Only in the browser

C Both server and browser

D Nowhere, it is ignored

The load function in +page.server.js runs only on the server to keep data fetching secure.

How do you export a server load function in +page.server.js?

A export async function clientLoad() {}

B export default function() {}

C export const serverLoad = () => {}

D export function load() {}

The server load function must be exported as load to be recognized by SvelteKit.

Which of these can you safely do inside a server load function?

A Use browser APIs like localStorage

B Access environment secrets

C Manipulate DOM elements

D Run client-side animations

Server load functions run on the server and can access environment secrets safely.

How do you access URL parameters inside a server load function?

A Using the <code>params</code> object passed as an argument

B Using <code>window.location</code>

C Using <code>document.querySelector</code>

D Using <code>fetch</code> API

The params object is provided to the server load function to access URL parameters.

What happens if you put a secret API key in client-side code instead of +page.server.js?

A It stays hidden automatically

B It runs faster

C It becomes visible to anyone using the app

D It encrypts itself

Client-side code is visible to users, so secrets placed there can be exposed.

Explain what a server load function in +page.server.js does and why it is important.

Think about where the code runs and what it can access.

Describe how you would access URL parameters inside a server load function and why this is useful.

Consider how URLs can carry information for the page.

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