Writable stores in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a writable store in Svelte?

A writable store is a special object in Svelte that holds a value you can read and change. It lets components share and react to data changes easily.

beginner

How do you create a writable store in Svelte?

Use the <code>writable</code> function from <code>svelte/store</code>. For example: <br><code>import { writable } from 'svelte/store';<br>const count = writable(0);</code>

beginner

How do you update the value of a writable store?

You can use the set method to replace the value or the update method to change it based on the current value.
Example:
count.set(5);
count.update(n => n + 1);

beginner

How do components react to changes in a writable store?

Components subscribe to the store. When the store value changes, subscribed components automatically update their displayed data.

intermediate

What is the difference between subscribe and set in writable stores?

subscribe lets you listen to changes in the store value. set changes the store value.
Subscribe is for reading/reacting, set is for writing.

Which function creates a writable store in Svelte?

A store()

B readable()

C writable()

D derived()

The writable() function creates a store you can both read and write.

How do you change the value of a writable store?

A Using set() or update() methods

B Using subscribe() method

C Directly assigning a new value

D Using get() method

set() replaces the value, update() changes it based on current value.

What does the subscribe() method do in writable stores?

A Creates a new store

B Changes the store value

C Deletes the store

D Listens for changes in the store value

subscribe() lets components react when the store value changes.

Which import is needed to use writable stores in Svelte?

A import { writable } from 'svelte/store';

B import { writable } from 'svelte';

C import writable from 'svelte/store';

D import { store } from 'svelte/store';

Writable stores come from the 'svelte/store' module.

If you want to increase a count store by 1, which method is best?

A count.set(count + 1);

B count.update(n => n + 1);

C count.subscribe(n => n + 1);

D count.get() + 1;

update() safely changes the value based on current state.

Explain how writable stores help manage shared state in Svelte applications.

Think about how multiple parts of an app can see and change the same data.

Describe the difference between the set() and update() methods on writable stores.

One method needs the new value, the other uses a function to compute it.

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