Response helpers (json, error) in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the json() helper do in SvelteKit?

The json() helper creates a response with JSON content and the correct Content-Type header. It formats data so the client can easily read it as JSON.

beginner

How do you use the error() helper in SvelteKit?

You call error(status, message) to create an error response with a status code and message. This stops the request and shows an error page or message.

intermediate

Why is it better to use json() instead of manually creating a JSON response?

Because json() automatically sets the right headers and formats the data correctly, making your code cleaner and less error-prone.

intermediate

What happens if you use error(404, 'Not found') in a SvelteKit load function?

The request stops and SvelteKit shows a 404 error page with the message 'Not found'. This helps handle missing data or pages gracefully.

beginner

Can you customize the error message shown to users when using error()?

Yes, you provide a custom message as the second argument to error(). This message can explain what went wrong in simple words.

What header does the json() helper automatically set?

A Content-Type: text/html

B Authorization: Bearer token

C Content-Type: application/json

D Cache-Control: no-cache

The json() helper sets the Content-Type header to application/json so the client knows the response is JSON.

What does error(500, 'Server error') do in SvelteKit?

A Returns a JSON response with status 200

B Throws a 500 error with message 'Server error'

C Redirects to the home page

D Logs the error but continues loading

The error() helper throws an error response with the given status and message, stopping the request.

Which helper should you use to send JSON data from a SvelteKit endpoint?

A fetch()

B error()

C redirect()

D json()

Use json() to send JSON data with proper headers from an endpoint.

If you want to stop a request and show a 404 page, which helper do you use?

A error(404, 'Not found')

B throw new Error('Not found')

C redirect('/404')

D json()

Using error(404, 'Not found') stops the request and shows the 404 error page.

What type of value does json() expect as input?

A A JavaScript object or array

B A string of HTML

C A number only

D A boolean value

json() expects a JavaScript object or array to convert into JSON format.

Explain how and why you would use the json() helper in a SvelteKit endpoint.

Think about sending data from server to client in a clean way.

Describe the purpose of the error() helper and how it affects the user experience in SvelteKit.

Consider what happens when something goes wrong on the server.

Practice

1. What is the main purpose of the json() helper in SvelteKit server endpoints?

easy

A. To parse incoming JSON requests

B. To throw an HTTP error with a message

C. To send a JSON response back to the client

D. To redirect the user to another page

Solution

1. Step 1: Understand the role of json()

   The json() helper formats data as a JSON response for the client.

2. Step 2: Differentiate from other helpers

   Unlike error(), which throws HTTP errors, json() sends data successfully.

3. Final Answer:

   To send a JSON response back to the client -> Option C

4. Quick Check:

   json() sends JSON data [OK]

Hint: json() sends data; error() throws errors [OK]

Common Mistakes:

• Confusing json() with error()
• Thinking json() parses requests
• Assuming json() redirects

2. Which of the following is the correct way to return a JSON response with a status code 200 in a SvelteKit endpoint?

easy

A. return json({ message: 'Success' }, 200);

B. return error(200, { message: 'Success' });

C. return new Response({ message: 'Success' }, 200);

D. return json({ message: 'Success' }, { status: 200 });

Solution

1. Step 1: Check json() syntax

   The json() helper takes data as first argument and an options object with status as second.

2. Step 2: Validate options object usage

   return json({ message: 'Success' }, { status: 200 }); correctly uses { status: 200 } as the second argument.

3. Final Answer:

   return json({ message: 'Success' }, { status: 200 }); -> Option D

4. Quick Check:

   json(data, { status }) syntax [OK]

Hint: json() needs options object for status [OK]

Common Mistakes:

• Passing status as a separate argument without object
• Using error() to send success responses
• Using Response constructor incorrectly

3. Consider this SvelteKit endpoint code:

import { json, error } from '@sveltejs/kit';

export function GET() {
  const data = { user: 'Alice' };
  if (!data.user) {
    throw error(404, 'User not found');
  }
  return json(data);
}

What will be the HTTP status code and response if data.user is missing?

medium

A. Status 200 with empty JSON object

B. Status 404 with message 'User not found'

C. Status 500 with server error

D. Status 200 with user data

Solution

1. Step 1: Analyze the condition

   If data.user is missing (falsy), the code throws error(404, 'User not found').

2. Step 2: Understand error() behavior

   The error() helper throws an HTTP error with the given status and message.

3. Final Answer:

   Status 404 with message 'User not found' -> Option B

4. Quick Check:

   Missing user triggers error(404) [OK]

Hint: error() throws HTTP errors with status and message [OK]

Common Mistakes:

• Assuming json() runs even if error is thrown
• Thinking error() returns JSON data
• Confusing status codes for errors

4. Identify the error in this SvelteKit endpoint code snippet:

import { json, error } from '@sveltejs/kit';

export function POST() {
  const data = { success: true };
  return error(200, 'All good');
}

medium

A. Using error() with status 200 is incorrect

B. Missing return type annotation

C. POST function must be async

D. json() should be used instead of error() to send success

Solution

1. Step 1: Check status code validity

   The error() helper requires status codes 400-599. Using 200 triggers "Non-error status code 200".

2. Step 2: Correct helper for success

   For success (status 200), use json() instead of error().

3. Final Answer:

   Using error() with status 200 is incorrect -> Option A

4. Quick Check:

   Use json() for success, error() for errors [OK]

Hint: error() is for errors, not success responses [OK]

Common Mistakes:

• Using error() with 200 status
• Not returning json() for success
• Assuming error() can send normal data

5. You want to create a SvelteKit endpoint that returns a JSON list of items but throws a 400 error if the list is empty. Which code snippet correctly implements this behavior?

hard

A. export function GET() {
const items = [];
if (items.length === 0) {
throw error(400, 'No items found');
}
return json(items);
}

B. export function GET() {
const items = [];
if (!items) {
return error(400, 'No items found');
}
return json(items);
}

C. export function GET() {
const items = [];
if (items.length === 0) {
return json({ error: 'No items found' }, { status: 400 });
}
return json(items);
}

D. export function GET() {
const items = [];
if (items.length === 0) {
throw json({ error: 'No items found' }, { status: 400 });
}
return json(items);
}

Solution

1. Step 1: Check error throwing for empty list

   export function GET() { const items = []; if (items.length === 0) { throw error(400, 'No items found'); } return json(items); } correctly throws error(400, 'No items found') when the list is empty.

2. Step 2: Validate response for non-empty list

   It returns json(items) properly when items exist.

3. Final Answer:

   export function GET() { const items = []; if (items.length === 0) { throw error(400, 'No items found'); } return json(items); } -> Option A

4. Quick Check:

   Throw error() for HTTP errors, json() for data [OK]

Hint: Throw error() for bad status, json() for good data [OK]

Common Mistakes:

• Returning error() instead of throwing it
• Using json() with error status instead of throwing error()
• Throwing json() which is invalid