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

Recall & Review

beginner

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

The +page.js file defines a page load function that runs before the page renders. It fetches data or prepares props for the page component.

beginner

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

You export a named async function called <code>load</code>. For example: <pre>export async function load({ params, fetch }) { /* ... */ }</pre>

intermediate

What arguments does the load function receive in +page.js?

It receives an object with properties like params (route parameters), fetch (to make requests), url (current URL), and route (route info).

beginner

What should the load function return in +page.js?

It should return an object with data that becomes props for the page component. For example:

return { user, posts }

intermediate

Can the load function in +page.js run on the server, client, or both?

By default, it runs on the server during initial page load. It can also run on the client during navigation between pages.

What does the load function in +page.js primarily do?

A Defines CSS styles for the page

B Fetches data before the page renders

C Handles user input events

D Manages routing configuration

The load function fetches or prepares data before the page renders.

Which of these is NOT a parameter passed to the load function?

A document

B fetch

C params

D url

The document object is not passed to the load function.

What must the load function return?

A Nothing, it returns void

B A string with HTML content

C A boolean indicating success

D An object with data for the page

The load function returns an object whose properties become props for the page.

Where does the load function run when you first visit a page?

A Only on the server

B Only in the browser console

C On both server and client simultaneously

D Only on the client

On initial page load, the load function runs on the server.

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

A export default function load() {}

B function load() {}

C export async function load() {}

D module.exports = load

You export an async named function called load using ES module syntax.

Explain how the load function in +page.js helps prepare data for a SvelteKit page.

Think about when and why you need data ready before showing a page.

Describe the difference in where the load function runs during initial page load versus client-side navigation.

Consider server rendering and client navigation separately.

Practice

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

easy

A. To fetch data or prepare information before the page is shown

B. To style the page with CSS

C. To handle user clicks on buttons

D. To define routes for the application

Solution

1. Step 1: Understand the role of load in SvelteKit

   The load function runs before the page renders to get data or do setup.

2. Step 2: Match the purpose with options

   Only To fetch data or prepare information before the page is shown describes fetching or preparing data before showing the page.

3. Final Answer:

   To fetch data or prepare information before the page is shown -> Option A

4. Quick Check:

   Page load functions fetch data before rendering [OK]

Hint: Load functions run before page shows to get data [OK]

Common Mistakes:

• Confusing load with styling or event handling
• Thinking load defines routes
• Assuming load runs after page renders

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

easy

A. export default function load() { return { props: { message: 'Hi' } }; }

B. function load() { return { message: 'Hi' }; }

C. export function load() { return { props: { message: 'Hi' } }; }

D. export const load = () => { return { message: 'Hi' }; };

Solution

1. Step 1: Recall correct export syntax for load

   In SvelteKit, load is exported as a named export, usually a const arrow function returning data without wrapping in props.

2. Step 2: Check each option

   export const load = () => { return { message: 'Hi' }; }; exports load as a named const returning an object with data directly, which is correct. export function load() { return { props: { message: 'Hi' } }; } wrongly uses props key, function load() { return { message: 'Hi' }; } lacks export, export default function load() { return { props: { message: 'Hi' } }; } uses default export which is incorrect.

3. Final Answer:

   export const load = () => { return { message: 'Hi' }; }; -> Option D

4. Quick Check:

   Named export const load returns data object [OK]

Hint: Use named export const load returning data object [OK]

Common Mistakes:

• Using default export instead of named export
• Returning data inside a props object
• Not exporting the load function

3. Given this +page.js code, what will be the value of data.greeting inside the page component?

export const load = () => {
  return { greeting: 'Hello, Svelte!' };
};

medium

A. undefined

B. 'Hello, Svelte!'

C. { greeting: 'Hello, Svelte!' }

D. An error occurs

Solution

1. Step 1: Understand what load returns

   The load function returns an object with a key greeting and value 'Hello, Svelte!'.

2. Step 2: How data is passed to the page

   The returned object keys become properties on the data object in the page component. So data.greeting equals 'Hello, Svelte!'.

3. Final Answer:

   'Hello, Svelte!' -> Option B

4. Quick Check:

   Returned keys become data props [OK]

Hint: Returned object keys become data properties [OK]

Common Mistakes:

• Expecting the whole object as data.greeting
• Thinking load returns props key
• Assuming undefined if not wrapped in props

4. Identify the error in this +page.js load function:

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

medium

A. The returned data must be wrapped in an object

B. fetch cannot be used inside load

C. The function should not be async

D. Missing export keyword

Solution

1. Step 1: Check the return value of load

   The load function must return an object whose keys become props for the page. Returning raw data (like an array or primitive) is invalid.

2. Step 2: Identify the fix

   Wrap the fetched data inside an object, e.g., return { data }, so the page receives it properly.

3. Final Answer:

   The returned data must be wrapped in an object -> Option A

4. Quick Check:

   Load returns object with keys for props [OK]

Hint: Always return an object from load function [OK]

Common Mistakes:

• Returning raw data instead of object
• Forgetting to export load
• Thinking fetch is disallowed in load

5. You want to redirect users to the login page if they are not authenticated inside a +page.js load function. Which code snippet correctly does this?

hard

A. import { redirect } from '@sveltejs/kit';

export const load = () => {
const user = null;
if (!user) return redirect(302, '/login');
return { user };
};

B. export const load = () => {
const user = null;
if (!user) return '/login';
return { user };
};

C. import { redirect } from '@sveltejs/kit';

export const load = () => {
const user = null;
if (!user) throw redirect(302, '/login');
return { user };
};

D. export const load = () => {
const user = null;
if (!user) window.location.href = '/login';
return { user };
};

Solution

1. Step 1: Understand how to redirect in load

   SvelteKit uses the redirect function from '@sveltejs/kit' which throws an exception to redirect the user.

2. Step 2: Check each option

   import { redirect } from '@sveltejs/kit'; export const load = () => { const user = null; if (!user) throw redirect(302, '/login'); return { user }; }; correctly imports redirect and throws it when user is not authenticated. export const load = () => { const user = null; if (!user) return '/login'; return { user }; }; returns a string which is invalid. export const load = () => { const user = null; if (!user) window.location.href = '/login'; return { user }; }; uses browser redirect which won't work server-side. import { redirect } from '@sveltejs/kit'; export const load = () => { const user = null; if (!user) return redirect(302, '/login'); return { user }; }; returns redirect instead of throwing it, which is incorrect.

3. Final Answer:

   import { redirect } from '@sveltejs/kit'; export const load = () => { const user = null; if (!user) throw redirect(302, '/login'); return { user }; }; -> Option C

4. Quick Check:

   Throw redirect to redirect in load [OK]

Hint: Throw redirect() from load to redirect users [OK]

Common Mistakes:

• Returning redirect instead of throwing it
• Using window.location in load (server-side)
• Returning a string URL to redirect