Writable stores in Svelte - Step-by-Step Execution

Concept Flow - Writable stores

Create writable store

↓

Subscribe to store

↓

Read current value

↓

Update store value

↓

Notify subscribers

↓

Subscribers react to new value

This flow shows how a writable store is created, subscribed to, updated, and how subscribers get notified with the new value.

Execution Sample

Svelte

import { writable } from 'svelte/store';

const count = writable(0);

count.subscribe(value => console.log('Count:', value));

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

This code creates a writable store, subscribes to it to log changes, then sets and updates the store value.

Execution Table

Step	Action	Store Value Before	Store Value After	Subscribers Notified	Output
1	Create writable store with initial value 0	N/A	0	No
2	Subscribe to store	0	0	Yes	Count: 0
3	Set store value to 1	0	1	Yes	Count: 1
4	Update store value by adding 1	1	2	Yes	Count: 2
5	No more actions	2	2	No

💡 No more updates; store value remains 2 and subscribers have latest value.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
count	undefined	0	0	1	2	2

Key Moments - 3 Insights

Why do subscribers get the initial value immediately after subscribing?

Because in step 2, subscribing calls the subscriber function right away with the current store value (0), as shown in the output 'Count: 0'.

What is the difference between set() and update() methods?

set() replaces the store value directly (step 3), while update() takes a function to compute the new value from the old one (step 4), as seen in the store value changes.

Why does the subscriber get notified after every change?

Because writable stores notify all subscribers whenever the value changes, triggering the subscriber callback each time (steps 3 and 4 outputs).

Visual Quiz - 3 Questions

Look at the execution table, what is the store value right after subscribing at step 2?

A 1

B 0

C undefined

D 2

💡 Hint

Check the 'Store Value After' column at step 2 in the execution table.

At which step does the store value become 2?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Store Value After' column for each step in the execution table.

If we remove the subscribe call, what will happen to the output?

A Store value will not update

B Output will still log all changes

C No output will be logged

D Subscribers will be notified but no output

💡 Hint

Refer to the execution table rows where output is logged only after subscribing.

Concept Snapshot

Writable stores in Svelte:
- Created with writable(initialValue)
- Subscribe to get updates immediately and on changes
- Use set(value) to replace value
- Use update(fn) to change value based on current
- Subscribers run callback on every change

Full Transcript

Writable stores in Svelte let you hold a value that can change over time. You create one with writable and give it a starting value. When you subscribe, your function runs right away with the current value. When you call set, the store value changes and subscribers get notified. Update lets you change the value based on the old one. This way, your app reacts automatically to changes in the store.

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