Server routes (+server.js) in Svelte - Mini Project: Build & Apply

Create a Simple Svelte Server Route with server.js

📖 Scenario: You are building a small web app using SvelteKit. You want to create a server route that returns a list of fruits as JSON data. This will help your frontend show the fruit list dynamically.

🎯 Goal: Build a SvelteKit server route using +server.js that returns a JSON response with a list of fruits.

📋 What You'll Learn

Create a server route file named +server.js

Define a GET handler function that returns a JSON response

Use an array of fruits as the data source

Return the fruits array in the response body with status 200

💡 Why This Matters

🌍 Real World

Server routes in SvelteKit let you build backend APIs easily inside your frontend project. This is useful for fetching data, handling forms, or connecting to databases.

💼 Career

Knowing how to create server routes is essential for full-stack SvelteKit developers. It helps you build dynamic web apps that communicate between frontend and backend smoothly.

1

Set up the fruits data array

Create a constant array called fruits with these exact string values: "apple", "banana", "cherry".

Svelte

// Your code here
// Create the fruits array with exact values

Hint

Use const fruits = ["apple", "banana", "cherry"] to create the array.

2

Create the GET handler function

Add an exported async function named GET that takes a single parameter {} (an empty object).

Svelte

const fruits = ["apple", "banana", "cherry"]

// Export an async GET function with parameter {} here
// Your code here

Hint

Write export async function GET({}) { } to define the route handler.

3

Return the fruits array as JSON response

Inside the GET function, return a new Response object with the JSON stringified fruits array as the body, and set the header Content-Type to application/json. Use status code 200.

Svelte

const fruits = ["apple", "banana", "cherry"]

export async function GET({}) {
  // Return JSON response with fruits array
  // Your code here
}

Hint

Use return new Response(JSON.stringify(fruits), { status: 200, headers: { 'Content-Type': 'application/json' } }).

4

Complete the +server.js file

Ensure the entire +server.js file contains the fruits array and the exported async GET function returning the JSON response exactly as shown.

Svelte

const fruits = ["apple", "banana", "cherry"]

export async function GET({}) {
  return new Response(JSON.stringify(fruits), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  })
}

// File complete

Hint

Check that the file has the fruits array and the GET function returning the JSON response.

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