Store contract (subscribe method) in Svelte - Step-by-Step Execution

Concept Flow - Store contract (subscribe method)

Create Store

↓

Call subscribe(callback)

↓

Add callback to subscribers list

↓

Immediately call callback with current value

↓

Return unsubscribe function

↓

When unsubscribe called -> Remove callback

↓

When store value changes -> Call all subscribers

This flow shows how subscribing to a Svelte store adds a callback, calls it immediately with the current value, and returns an unsubscribe function to stop updates.

Execution Sample

Svelte

import { writable } from 'svelte/store';
const count = writable(0);
const unsubscribe = count.subscribe(value => {
  console.log('Count:', value);
});
unsubscribe();

This code creates a writable store, subscribes to it logging the value, then unsubscribes to stop updates.

Execution Table

Step	Action	Subscribers List	Callback Called With	Return Value
1	Create writable store with initial value 0	[]	N/A	Store object
2	Call subscribe with callback	[(callback)]	0 (immediate call)	unsubscribe function
3	Store value changes to 1	[(callback)]	1 (callback called)	N/A
4	Call unsubscribe function	[]	N/A	N/A
5	Store value changes to 2	[]	No callback called	N/A

💡 No more subscribers after unsubscribe, so no callbacks called on value changes.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
subscribers	[]	[(callback)]	[(callback)]	[]	[]
store value	0	0	1	1	2

Key Moments - 3 Insights

Why is the callback called immediately when subscribing?

Because the subscribe method calls the callback right away with the current store value to give the subscriber the latest state (see execution_table step 2).

What happens if you don't call the unsubscribe function?

The callback stays in the subscribers list and will be called on every store value change, potentially causing memory leaks or unwanted updates (see execution_table steps 3 and 5).

Can multiple subscribers listen to the same store?

Yes, each subscribe call adds a callback to the subscribers list, all of which get called on value changes.

Visual Quiz - 3 Questions

Look at the execution table, what is the subscribers list after step 4?

A [(callback)]

B []

C [undefined]

D null

💡 Hint

Check the 'Subscribers List' column at step 4 in the execution_table.

At which step does the callback first get called with the value 1?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Callback Called With' column in the execution_table.

If you never call unsubscribe, what happens when the store value changes to 2?

A Subscribers list becomes empty

B Callback is not called

C Callback is called with 2

D Store value resets to 0

💡 Hint

Refer to the 'Callback Called With' and 'Subscribers List' columns at step 5.

Concept Snapshot

Svelte store subscribe method:
- subscribe(callback) adds callback to subscribers
- Calls callback immediately with current value
- Returns unsubscribe function to remove callback
- On store value change, all subscribers are called
- Unsubscribe stops updates to that callback

Full Transcript

In Svelte, stores have a subscribe method that lets you listen for changes. When you subscribe, your callback is added to a list and called immediately with the current value. You get back an unsubscribe function to stop listening. When the store value changes, all subscribed callbacks run with the new value. If you unsubscribe, your callback is removed and won't be called anymore. This helps keep your app efficient and reactive.

Practice

1. What does the subscribe method of a Svelte store do?

easy

A. It resets the store to its initial value.

B. It updates the store value directly.

C. It deletes the store permanently.

D. It lets you watch store changes and react immediately.

Solution

1. Step 1: Understand the purpose of subscribe

   The subscribe method allows you to listen to changes in the store's value.

2. Step 2: Identify what subscribe enables

   It lets your code react immediately whenever the store value changes.

3. Final Answer:

   It lets you watch store changes and react immediately. -> Option D

4. Quick Check:

   subscribe watches changes = C [OK]

Hint: Remember: subscribe means listen and react to changes [OK]

Common Mistakes:

• Thinking subscribe changes the store value
• Confusing subscribe with reset or delete
• Assuming subscribe returns the store value directly

2. Which of the following is the correct syntax to subscribe to a Svelte store named count?

easy

A. count.subscribe = value => console.log(value);

B. count.subscribe(value => console.log(value));

C. subscribe(count, value => console.log(value));

D. count.onSubscribe(value => console.log(value));

Solution

1. Step 1: Recall the store subscription syntax

   The correct way is to call subscribe as a method on the store with a callback.

2. Step 2: Match the syntax to the options

   count.subscribe(value => console.log(value)); correctly calls count.subscribe with a function to receive updates.

3. Final Answer:

   count.subscribe(value => console.log(value)); -> Option B

4. Quick Check:

   Method call subscribe with callback = A [OK]

Hint: subscribe is a method: store.subscribe(callback) [OK]

Common Mistakes:

• Assigning subscribe instead of calling it
• Using a non-existent method like onSubscribe
• Calling subscribe as a standalone function

3. Given the code below, what will be logged to the console?

import { writable } from 'svelte/store';
const count = writable(0);
const unsubscribe = count.subscribe(value => console.log(value));
count.set(5);
unsubscribe();
count.set(10);

medium

A. 0, 5

B. 0, 5, 10

C. 5, 10

D. 10

Solution

1. Step 1: Understand initial subscription behavior

   When subscribing, the callback immediately receives the current value, which is 0.

2. Step 2: Trace subsequent updates and unsubscription

   After setting to 5, the callback logs 5. Then unsubscribe() stops listening, so setting to 10 logs nothing.

3. Final Answer:

   0, 5 -> Option A

4. Quick Check:

   subscribe logs initial + updates until unsubscribe = B [OK]

Hint: subscribe logs current value immediately, stops after unsubscribe [OK]

Common Mistakes:

• Expecting logs after unsubscribe
• Missing initial immediate callback call
• Confusing set with update method

4. What is wrong with this code snippet?

import { writable } from 'svelte/store';
const count = writable(0);
count.subscribe(value => console.log(value));
count.unsubscribe();

medium

A. You must import subscribe separately from the store.

B. The subscribe callback must return a value.

C. You cannot call unsubscribe directly on the store.

D. The store must be initialized with a string, not a number.

Solution

1. Step 1: Understand unsubscribe usage

   The subscribe method returns an unsubscribe function to stop listening.

2. Step 2: Identify the error in calling unsubscribe

   Calling unsubscribe directly on the store is invalid; you must call the function returned by subscribe.

3. Final Answer:

   You cannot call unsubscribe directly on the store. -> Option C

4. Quick Check:

   unsubscribe is returned by subscribe, not a store method [OK]

Hint: unsubscribe comes from subscribe, not store itself [OK]

Common Mistakes:

• Calling unsubscribe on the store instead of returned function
• Expecting subscribe callback to return a value
• Confusing store initialization types

5. You want to create a custom store that logs every subscription and unsubscription. Which of the following correctly implements the subscribe method to follow the store contract?

hard

A. function customStore() {
let value = 0;
return {
subscribe(run) {
console.log('subscribed');
run(value);
return () => console.log('unsubscribed');
}
};
}

B. function customStore() {
let value = 0;
return {
subscribe(run) {
console.log('subscribed');
return () => console.log('unsubscribed');
}
};
}

C. function customStore() {
let value = 0;
return {
subscribe() {
console.log('subscribed');
return () => console.log('unsubscribed');
}
};
}

D. function customStore() {
let value = 0;
return {
subscribe(run) {
run(value);
console.log('subscribed');
}
};
}

Solution

1. Step 1: Check the subscribe method signature

   The subscribe method must accept a callback run and call it immediately with the current value.

2. Step 2: Verify logging and return of unsubscribe

   function customStore() { let value = 0; return { subscribe(run) { console.log('subscribed'); run(value); return () => console.log('unsubscribed'); } }; } logs 'subscribed' before calling run(value) and returns a function that logs 'unsubscribed'.

3. Step 3: Confirm distractors are invalid

   B misses calling run(value), C lacks run parameter, D misses returning the unsubscribe function.

4. Final Answer:

   correctly implements subscribe with logging and unsubscribe function -> Option A

5. Quick Check:

   subscribe calls run(value) and returns unsubscribe function [OK]

Hint: subscribe calls run(value) then returns unsubscribe function [OK]

Common Mistakes:

• Not calling run with current value
• Missing the unsubscribe function return
• Logging after calling run instead of before