Server routes (+server.js) in Svelte

Introduction

Server routes let your app handle requests like getting or sending data. They work like helpers that listen and respond to users or other apps.

You want to get data from a database and send it to your webpage.

You need to save user input from a form to a server.

You want to create an API that other apps can use.

You want to handle login or signup actions securely.

You want to serve files or data without showing them in the browser URL.

Syntax

Svelte

export async function GET() {
  // handle GET request
  return new Response('Hello from GET');
}

export async function POST({ request }) {
  // handle POST request
  const data = await request.json();
  return new Response(JSON.stringify({ message: 'Data received', data }), {
    headers: { 'Content-Type': 'application/json' }
  });
}

Each HTTP method (GET, POST, etc.) is a separate exported async function.

The handler receives an object with properties like request, params, url, and helpers.

Examples

A simple GET route that returns plain text.

Svelte

export async function GET() {
  return new Response('Hello world');
}

A POST route that reads JSON data sent by the client and responds with JSON.

Svelte

export async function POST({ request }) {
  const data = await request.json();
  return new Response(JSON.stringify({ received: data }), {
    headers: { 'Content-Type': 'application/json' }
  });
}

A DELETE route that confirms deletion.

Svelte

export async function DELETE() {
  return new Response('Deleted item');
}

Sample Program

This server route handles GET and POST requests at /api/message. GET returns a simple text message. POST reads JSON with a text property and replies with a JSON message.

Svelte

// src/routes/api/message/+server.js

export async function GET() {
  return new Response('Hello from server route!');
}

export async function POST({ request }) {
  const data = await request.json();
  const reply = `You sent: ${data.text}`;
  return new Response(JSON.stringify({ reply }), {
    headers: { 'Content-Type': 'application/json' }
  });
}

Important Notes

Server routes live in folders named with a +server.js file inside src/routes.

Use request (destructured from the handler function argument) to access request details like headers and body.

Always set proper Content-Type headers when returning JSON.

Summary

Server routes let your app respond to HTTP requests with code.

Each HTTP method is a separate exported async function in +server.js.

Use server routes to get, send, or process data securely on the server side.

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