Response helpers (json, error) in Svelte - Step-by-Step Execution

Concept Flow - Response helpers (json, error)

Request received

↓

Process data

↓

If success

yes→Use json() helper

↓

Send JSON response

↓

If error

yes→Use error() helper

↓

Send error response

This flow shows how a SvelteKit endpoint uses json() to send data on success, or error() to send error responses.

Execution Sample

Svelte

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

export function GET() {
  const data = { message: 'Hello' };
  if (!data) throw error(404, 'Not found');
  return json(data);
}

This code returns JSON data if found, otherwise throws a 404 error using SvelteKit helpers.

Execution Table

Step	Action	Condition	Helper Used	Response Sent
1	GET request received	N/A	N/A	N/A
2	Check if data exists	data is { message: 'Hello' }	N/A	N/A
3	Data exists, no error thrown	true	N/A	N/A
4	Return json(data)	N/A	json()	JSON { message: 'Hello' } sent
5	End of function	N/A	N/A	Response complete

💡 Function ends after sending JSON response with json() helper

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
data	{ message: 'Hello' }	{ message: 'Hello' }	{ message: 'Hello' }	{ message: 'Hello' }

Key Moments - 2 Insights

Why do we use json() instead of returning the object directly?

json() formats the data properly as a JSON response with correct headers, as shown in step 4 of the execution_table.

What happens if data is missing or invalid?

The error() helper is used to throw an HTTP error with status and message, stopping normal response flow (not shown in this example).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what helper is used to send the response?

A json()

B error()

C fetch()

D redirect()

💡 Hint

Check the 'Helper Used' column at step 4 in the execution_table

At which step does the code check if data exists?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Condition' column to find where data is checked

If data was null, which helper would be used to respond?

A json()

B error()

C redirect()

D fetch()

💡 Hint

Recall the key_moments about error handling when data is missing

Concept Snapshot

SvelteKit response helpers:
- Use json(data) to send JSON with correct headers
- Use error(status, message) to throw HTTP errors
- json() returns a Response with JSON body
- error() stops execution and sends error response
- Helps keep endpoint code clean and consistent

Full Transcript

In SvelteKit endpoints, you handle responses using helpers. When your data is ready and valid, you use the json() helper to send it back as a JSON response with proper headers. If something goes wrong, like missing data, you use the error() helper to throw an HTTP error with a status code and message. This stops normal processing and sends an error response. The flow starts with receiving a request, checking data, then either returning json(data) or throwing error(status, message). This keeps your code simple and your responses correct.

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