Server routes (+server.js) in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this +server.js GET route return?

Consider this SvelteKit +server.js file with a GET handler. What is the JSON response when you fetch this route?

Svelte

export async function GET() {
  return new Response(JSON.stringify({ message: 'Hello from server' }), {
    headers: { 'Content-Type': 'application/json' }
  });
}

A {"error":"Not found"}

B undefined

C "Hello from server"

D {"message":"Hello from server"}

💡 Hint

The GET function returns a Response with JSON string and content-type header.

The GET handler returns a Response object with a JSON string containing the message key. The client receives the JSON object {"message":"Hello from server"}.

📝 Syntax

intermediate

Which +server.js POST handler syntax is correct?

You want to create a POST handler in +server.js that reads JSON from the request body. Which option is syntactically correct?

A

export async function POST(request) {
  const data = await request.json();
  return new Response(JSON.stringify(data));
}

B

export async function POST({ request }) {
  const data = await request.json();
  return new Response(JSON.stringify(data));
}

C

export async function POST(req) {
  const data = req.json();
  return new Response(JSON.stringify(data));
}

D

export async function POST({ req }) {
  const data = await req.json();
  return new Response(JSON.stringify(data));
}

💡 Hint

SvelteKit passes an object with a request property to the handler.

The POST handler receives an object with a request property. You must destructure it as { request } and await request.json() to parse the body.

🔧 Debug

advanced

Why does this +server.js GET handler cause a runtime error?

This GET handler throws an error when called. What is the cause?

Svelte

export async function GET() {
  return {
    body: { message: 'Hi' }
  };
}

A The handler must return a Response object, not a plain object with body.

B The body property must be a string, not an object.

C The function must be named get, not GET.

D The handler must be synchronous, not async.

💡 Hint

SvelteKit expects a Response or a special object shape from server route handlers.

In SvelteKit +server.js, handlers must return a Response object or use the new Response helpers. Returning a plain object with body causes a runtime error.

❓ state_output

advanced

What is the response body after this POST handler runs?

Given this +server.js POST handler, what JSON does the client receive?

Svelte

export async function POST({ request }) {
  const { name } = await request.json();
  return new Response(JSON.stringify({ greeting: `Hello, ${name}!` }), {
    headers: { 'Content-Type': 'application/json' }
  });
}

A {"name":"Alice"}

B {"greeting":"Hello, undefined!"}

C {"greeting":"Hello, Alice!"}

D {"error":"Missing name"}

💡 Hint

The handler reads 'name' from the JSON body and returns a greeting string.

The handler extracts 'name' from the request JSON and returns a greeting message with that name in JSON format.

🧠 Conceptual

expert

Which statement about +server.js route handlers is TRUE?

Select the correct statement about how +server.js route handlers work in SvelteKit.

A Handlers must always return a Response object; returning plain objects causes errors.

B Handlers can return plain objects with a 'body' property and SvelteKit converts them automatically.

C Handlers can only handle GET and POST methods; other HTTP methods are not supported.

D Handlers run only on the client side and cannot access server resources.

💡 Hint

Think about the expected return type of server route handlers.

SvelteKit expects server route handlers to return Response objects or use the Response helpers. Returning plain objects is not supported and causes errors.

Practice

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

easy

A. To style the application using CSS

B. To define client-side UI components

C. To manage database connections directly in the browser

D. To handle server-side HTTP requests like GET or POST

Solution

1. Step 1: Understand the role of +server.js

   This file is used to write server-side code that responds to HTTP requests.

2. Step 2: Differentiate client and server roles

   Client-side UI components and styling are handled elsewhere, not in +server.js.

3. Final Answer:

   To handle server-side HTTP requests like GET or POST -> Option D

4. Quick Check:

   Server routes = handle HTTP requests [OK]

Hint: Remember: +server.js is for server code only [OK]

Common Mistakes:

• Confusing +server.js with client components
• Thinking CSS or styling goes in +server.js
• Assuming database code runs in the browser

2. Which of the following is the correct way to export a GET handler in a +server.js file?

easy

A. export async function get() { return { body: { message: 'Hello' } }; }

B. export function get() { return { body: { message: 'Hello' } }; }

C. export async function GET() { return { body: { message: 'Hello' } }; }

D. export async get() { return { body: { message: 'Hello' } }; }

Solution

1. Step 1: Recall SvelteKit server route syntax

   Handlers must be exported as async functions named in uppercase HTTP methods, e.g., GET.

2. Step 2: Check syntax correctness

   Only export async function GET() { return { body: { message: 'Hello' } }; } uses export async function GET() which is correct syntax.

3. Final Answer:

   export async function GET() { return { body: { message: 'Hello' } }; } -> Option C

4. Quick Check:

   Use uppercase HTTP method names for exports [OK]

Hint: Export async functions named uppercase HTTP methods [OK]

Common Mistakes:

• Using lowercase method names like 'get' instead of 'GET'
• Omitting async keyword for async handlers
• Incorrect export syntax without 'function'

3. Given this +server.js code, what will be the JSON response body when a GET request is made?

export async function GET() {
  return {
    status: 200,
    body: { success: true, data: [1, 2, 3] }
  };
}

medium

A. {"success":true,"data":[1,2,3]}

B. {"status":200,"body":{"success":true,"data":[1,2,3]}}

C. [1, 2, 3]

D. An error because body must be a string

Solution

1. Step 1: Understand the returned object structure

   The handler returns an object with status and body. The body is the JSON data sent to the client.

2. Step 2: Identify the actual response body content

   The client receives the body content serialized as JSON: {"success":true,"data":[1,2,3]}.

3. Final Answer:

   {"success":true,"data":[1,2,3]} -> Option A

4. Quick Check:

   Response body = JSON data object [OK]

Hint: Response body is the JSON object inside returned body [OK]

Common Mistakes:

• Confusing the whole return object with the response body
• Expecting raw arrays or strings instead of JSON objects
• Thinking status code is part of JSON body

4. What is wrong with this +server.js POST handler?

export async function POST(request) {
  const data = await request.json();
  return { status: 201, body: { received: data } };
}

medium

A. Nothing is wrong; this is correct syntax

B. The request parameter is missing the 'event' wrapper

C. The handler must return a Response object, not a plain object

D. The function should be named post in lowercase

Solution

1. Step 1: Check handler parameter

   The POST handler receives an event object containing request. The code names it request but calls request.json(), which fails because the event object has no json() method; use event.request.json() or destructure { request }.

2. Step 2: Identify the correct fix

   The parameter needs destructuring like POST({ request }) or renaming to event with event.request. The request parameter is missing the 'event' wrapper correctly identifies the missing 'event' wrapper.

3. Final Answer:

   The request parameter is missing the 'event' wrapper -> Option B

4. Quick Check:

   Destructure { request } from event [OK]

Hint: Destructure { request } from event parameter [OK]

Common Mistakes:

• Using lowercase method names instead of uppercase
• Expecting a Response object instead of plain object
• Misunderstanding request parameter structure

5. You want to create a +server.js route that accepts a POST request with JSON data, validates that the JSON has a non-empty name string, and returns a 400 error if invalid. Which code snippet correctly implements this?

hard

A. export async function POST({ request }) {
const data = await request.json();
if (!data.name || typeof data.name !== 'string' || data.name.trim() === '') {
return { status: 400, body: { error: 'Name is required' } };
}
return { status: 200, body: { message: `Hello, ${data.name}` } };
}

B. export async function POST(request) {
const data = await request.json();
if (!data.name) {
throw new Error('Name missing');
}
return new Response(JSON.stringify({ message: `Hello, ${data.name}` }), { status: 200 });
}

C. export async function POST({ request }) {
const data = await request.json();
if (data.name === '') {
return { status: 500, body: { error: 'Empty name' } };
}
return { status: 200, body: { message: `Hi ${data.name}` } };
}

D. export async function POST({ request }) {
const data = await request.json();
if (!data.name) {
return { status: 404, body: { error: 'Name not found' } };
}
return { status: 200, body: { message: `Welcome, ${data.name}` } };
}

Solution

1. Step 1: Validate the 'name' field properly

   The correct snippet destructures { request }, checks if name exists, is a string, and non-empty after trim(), returning 400 if invalid.

2. Step 2: Confirm status codes and completeness

   It uses 400 for validation errors and 200 for success; others use wrong parameters, incomplete checks, wrong statuses (500/404), or throw errors.

3. Final Answer:

   The snippet with full validation (exists, string, trimmed non-empty) and 400 status -> Option A

4. Quick Check:

   Validate input and return 400 if invalid [OK]

Hint: Check for empty or missing fields, return 400 for bad input [OK]

Common Mistakes:

• Returning wrong status codes like 404 or 500 for validation errors
• Throwing errors instead of returning HTTP responses
• Not trimming strings before checking emptiness