Writable stores in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the writable function from Svelte.

Svelte

import { [1] } from 'svelte/store';

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

Awritable

Bderived

Creadable

Dstore

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readable' instead of 'writable' because readable stores cannot be changed.

Trying to import 'store' which is not a function.

The writable function is used to create writable stores in Svelte.

2fill in blank

medium

Complete the code to create a writable store with initial value 0.

Svelte

const count = [1](0);

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

Aderived

Bwritable

Creadable

Dstore

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readable' which creates a store that cannot be changed.

Using 'derived' which creates a store based on others.

Use writable(0) to create a store starting at 0 that you can update.

3fill in blank

hard

Fix the error in the code to update the writable store value.

Svelte

count.[1](n => n + 1);

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

Aupdate

Bset

Csubscribe

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' which replaces the value directly instead of updating based on current.

Using 'subscribe' which is for listening, not changing.

The update method lets you change the store value based on the current value.

4fill in blank

hard

Fill both blanks to subscribe to the store and log its value.

Svelte

const unsubscribe = count.[1](value => {
  console.[2](value);
});

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

Asubscribe

Blog

Cwarn

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'subscribe' to listen to changes.

Using 'warn' instead of 'log' to print the value.

Use subscribe to listen to store changes and console.log to print the value.

5fill in blank

hard

Fill all three blanks to create a writable store, update it, and set a new value.

Svelte

import { [1] } from 'svelte/store';

const score = [2](10);
score.[3](20);

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

Awritable

Cset

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' instead of 'set' to assign a new value directly.

Importing 'readable' instead of 'writable'.

Import writable, create the store with writable(10), then use set(20) to change the value.

Practice

1. What is the main purpose of a writable store in Svelte?

easy

A. To style components dynamically

B. To hold data that can be changed and shared across components

C. To create static, unchangeable data

D. To handle routing between pages

Solution

1. Step 1: Understand writable store role

   Writable stores in Svelte are designed to hold data that can be changed and shared reactively.

2. Step 2: Compare options

   Only To hold data that can be changed and shared across components correctly describes writable stores as changeable and shareable data holders.

3. Final Answer:

   To hold data that can be changed and shared across components -> Option B

4. Quick Check:

   Writable store = changeable shared data [OK]

Hint: Writable stores hold changeable shared data [OK]

Common Mistakes:

• Confusing writable stores with static data
• Thinking writable stores handle styling or routing
• Assuming writable stores cannot be updated

2. Which of the following is the correct way to create a writable store in Svelte?

easy

A. import { writable } from 'svelte/store'; const count = writable(0);

B. import { readable } from 'svelte/store'; const count = readable(0);

C. const count = writable(0); without import

D. import { store } from 'svelte'; const count = store(0);

Solution

1. Step 1: Check import statement

   The writable store must be imported from 'svelte/store' using { writable }.

2. Step 2: Verify store creation syntax

   Creating a writable store uses writable(initialValue), so import { writable } from 'svelte/store'; const count = writable(0); is correct.

3. Final Answer:

   import { writable } from 'svelte/store'; const count = writable(0); -> Option A

4. Quick Check:

   Writable store import and creation = import { writable } from 'svelte/store'; const count = writable(0); [OK]

Hint: Writable stores need import { writable } from 'svelte/store' [OK]

Common Mistakes:

• Using readable instead of writable
• Forgetting to import writable
• Wrong import path or syntax

3. Given this code:

import { writable } from 'svelte/store';
const count = writable(5);
count.update(n => n + 3);
console.log($count);

What will be logged to the console?

medium

A. Error: $count is undefined

B. 8

C. 3

D. 5

Solution

1. Step 1: Understand store update

   The update method changes the store value from 5 to 8 by adding 3.

2. Step 2: Check usage of $count outside component

   The $ prefix to access store value works only inside Svelte components, not in plain JS code like console.log.

3. Final Answer:

   Error: $count is undefined -> Option A

4. Quick Check:

   $ prefix only works inside components [OK]

Hint: Use $ only inside components, not in plain JS [OK]

Common Mistakes:

• Assuming $count works outside components
• Ignoring that update changes value
• Expecting console.log to show store value directly

4. What is wrong with this code snippet?

import { writable } from 'svelte/store';
const count = writable(0);
count.set(count + 1);

medium

A. The initial value must be a string

B. You cannot use set on writable stores

C. Missing import for set function

D. count + 1 is invalid because count is a store, not a number

Solution

1. Step 1: Identify what count represents

   count is a writable store object, not a number, so count + 1 is invalid.

2. Step 2: Correct way to update store

   You must use count.update(n => n + 1) or count.set(value) with a number, not count + 1.

3. Final Answer:

   count + 1 is invalid because count is a store, not a number -> Option D

4. Quick Check:

   Store object + number is invalid [OK]

Hint: Use update or get value before set, not store + number [OK]

Common Mistakes:

• Trying to add number directly to store object
• Forgetting to use update for changes
• Assuming set needs import separately

5. You want to create a writable store that holds a number starting at 10. Then you want to double its value only if it is less than 50. Which code correctly does this?

hard

A. const num = writable(10);
num.set(20);

B. const num = writable(10);
num.set(n => n < 50 ? n * 2 : n);

C. const num = writable(10);
num.update(n => n < 50 ? n * 2 : n);

D. const num = writable(10);
num.update(n => n * 2);

Solution

1. Step 1: Create writable store with initial value 10

   Using writable(10) correctly sets the initial number.

2. Step 2: Use update with condition to double if less than 50

   update receives current value n and returns n*2 if n < 50, else n unchanged, matching requirement.

3. Step 3: Check other options

   const num = writable(10); num.set(n => n < 50 ? n * 2 : n); wrongly uses set with a function, which is invalid. const num = writable(10); num.update(n => n * 2); doubles unconditionally. const num = writable(10); num.set(20); sets fixed 20 ignoring condition.

4. Final Answer:

   const num = writable(10); num.update(n => n < 50 ? n * 2 : n); -> Option C

5. Quick Check:

   Use update with condition to change value [OK]

Hint: Use update(fn) with condition to change store value [OK]

Common Mistakes:

• Using set with a function instead of a value
• Not applying condition inside update
• Setting fixed value ignoring condition