Middleware patterns with hooks in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the hook function correctly.

Svelte

export const [1] = async ({ event, resolve }) => {
  return await resolve(event);
};

Drag options to blanks, or click blank then click option'

Ahandle

BuseEffect

ConMount

DbeforeUpdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using lifecycle hooks like useEffect which are React-specific.

Importing hooks that do not exist in SvelteKit.

The handle function is the correct hook to implement middleware in SvelteKit.

2fill in blank

medium

Complete the code to define a middleware that logs each request method.

Svelte

export const handle = async ({ event, resolve }) => {
  console.log(event.request.[1]);
  return await resolve(event);
};

Drag options to blanks, or click blank then click option'

Aurl

Bheaders

Cmethod

Dbody

Attempts:

3 left

💡 Hint

Common Mistakes

Using url instead of method to log the HTTP verb.

Trying to access body directly without parsing.

The method property of the request object gives the HTTP method like GET or POST.

3fill in blank

hard

Fix the error in the middleware to correctly modify the response headers.

Svelte

export const handle = async ({ event, resolve }) => {
  const response = await resolve(event);
  response.headers.set('[1]', 'no-store');
  return response;
};

Drag options to blanks, or click blank then click option'

Acache control

BCache-Control

Ccache_control

Dcachecontrol

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or underscored header names which are invalid.

Adding spaces in header names.

HTTP headers are case-insensitive but should be written in standard format like Cache-Control.

4fill in blank

hard

Fill both blanks to create middleware that blocks requests without a token header.

Svelte

export const handle = async ({ event, resolve }) => {
  const token = event.request.headers.get('[1]');
  if (!token) {
    return new Response('Unauthorized', { status: [2] });
  }
  return await resolve(event);
};

Drag options to blanks, or click blank then click option'

Aauthorization

B401

C403

Dtoken

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong header names like token.

Returning status 403 which means forbidden, not unauthorized.

The middleware checks the authorization header and returns status 401 if missing.

5fill in blank

hard

Fill all three blanks to create middleware that adds a custom header and logs the response status.

Svelte

export const handle = async ({ event, resolve }) => {
  const response = await resolve(event);
  response.headers.set('[1]', '[2]');
  console.log('Response status:', response.[3]);
  return response;
};

Drag options to blanks, or click blank then click option'

AX-Custom-Header

BSvelteKit

Cstatus

DstatusText

Attempts:

3 left

💡 Hint

Common Mistakes

Using statusText instead of status to log the status code.

Forgetting to set the header with the correct name.

The middleware sets a custom header X-Custom-Header with value SvelteKit and logs the numeric status of the response.

Practice

1. Which Svelte hook is best suited to run code right after a component appears on the page?

easy

A. onDestroy

B. onMount

C. afterUpdate

D. beforeUpdate

Solution

1. Step 1: Understand when onMount runs

   onMount runs once right after the component is added to the page.

2. Step 2: Compare with other hooks

   beforeUpdate and afterUpdate run on updates, onDestroy runs when component is removed.

3. Final Answer:

   onMount -> Option B

4. Quick Check:

   Run code after component appears = onMount [OK]

Hint: Use onMount for code after component loads [OK]

Common Mistakes:

• Confusing onMount with beforeUpdate
• Using onDestroy to run code on load
• Thinking afterUpdate runs only once

2. Which is the correct syntax to import and use the onDestroy hook in a Svelte component?

easy

A. import { onDestroy } from 'svelte/hooks'; onDestroy(() => { /* cleanup */ });

B. import onDestroy from 'svelte'; onDestroy(() => { /* cleanup */ });

C. import { onDestroy } from 'svelte'; onDestroy = () => { /* cleanup */ };

D. import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ });

Solution

1. Step 1: Check correct import syntax

   Svelte exports hooks like onDestroy as named exports from 'svelte'.

2. Step 2: Verify usage

   The hook is called as a function with a callback: onDestroy(() => { ... }).

3. Final Answer:

   import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ }); -> Option D

4. Quick Check:

   Named import + callback function = import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ }); [OK]

Hint: Use named import from 'svelte' and call with callback [OK]

Common Mistakes:

• Using default import instead of named import
• Importing from wrong path like 'svelte/hooks'
• Assigning hook to a variable instead of calling it

3. Given this Svelte code snippet, what will be logged to the console when the component updates?

import { beforeUpdate, afterUpdate } from 'svelte';

beforeUpdate(() => console.log('Before update'));
afterUpdate(() => console.log('After update'));

let count = 0;

function increment() {
  count += 1;
}

medium

A. After update only once after first render

B. Only Before update once when component mounts

C. Before update\nAfter update each time increment() is called

D. No logs because hooks are not called

Solution

1. Step 1: Understand when hooks run

   beforeUpdate runs before every DOM update, afterUpdate runs after every DOM update.

2. Step 2: Analyze increment effect

   Calling increment() changes count, triggering component update and both hooks logging messages.

3. Final Answer:

   Before update\nAfter update each time increment() is called -> Option C

4. Quick Check:

   Hooks run on every update = Before update\nAfter update each time increment() is called [OK]

Hint: Hooks run before and after every update triggered by state change [OK]

Common Mistakes:

• Thinking hooks run only once on mount
• Assuming logs happen without calling increment()
• Confusing beforeUpdate and afterUpdate timing

4. What is wrong with this Svelte code using onDestroy?

import { onDestroy } from 'svelte';

onDestroy = () => {
  console.log('Cleanup');
};

medium

A. Assigning a function to onDestroy instead of calling it

B. Missing import statement for onDestroy

C. Using arrow function instead of regular function

D. onDestroy should be used inside onMount

Solution

1. Step 1: Check how onDestroy is used

   The code assigns a function to onDestroy instead of calling it with a callback.

2. Step 2: Correct usage

   onDestroy must be called like onDestroy(() => { ... }) to register cleanup.

3. Final Answer:

   Assigning a function to onDestroy instead of calling it -> Option A

4. Quick Check:

   Use onDestroy(callback), not assignment [OK]

Hint: Call onDestroy with a callback, don't assign it [OK]

Common Mistakes:

• Assigning instead of calling the hook
• Thinking arrow functions are disallowed
• Believing onDestroy must be inside onMount

5. You want to log a message every time a Svelte component updates but only after the DOM has changed. Which hook should you use and why? Also, how would you ensure cleanup if needed?

hard

A. Use afterUpdate to log after DOM changes; use onDestroy for cleanup

B. Use beforeUpdate to log after DOM changes; no cleanup needed

C. Use onMount to log after DOM changes; cleanup inside afterUpdate

D. Use onDestroy to log after DOM changes; cleanup inside beforeUpdate

Solution

1. Step 1: Identify hook for after DOM update

   afterUpdate runs after the DOM updates, perfect for logging post-change.

2. Step 2: Determine cleanup hook

   onDestroy runs when component is removed, ideal for cleanup tasks.

3. Final Answer:

   Use afterUpdate to log after DOM changes; use onDestroy for cleanup -> Option A

4. Quick Check:

   Log after DOM update = afterUpdate; cleanup = onDestroy [OK]

Hint: Log in afterUpdate; cleanup in onDestroy [OK]

Common Mistakes:

• Using beforeUpdate to log after DOM changes
• Trying to cleanup inside afterUpdate
• Confusing onMount with update hooks