Store contract (subscribe method) in Svelte - Mini Project: Build & Apply

Create a Simple Svelte Store with subscribe Method

📖 Scenario: You are building a small Svelte app that needs to share a number value across components. To do this, you will create a custom store that follows the Svelte store contract by implementing a subscribe method.

🎯 Goal: Build a custom Svelte store object with a subscribe method that allows components to listen for changes in the store's value.

📋 What You'll Learn

Create a store object with an initial value of 0

Add a subscribe method to the store object

Implement a simple way to notify subscribers when the value changes

Add a method to update the store's value

💡 Why This Matters

🌍 Real World

Custom stores are useful when you want to share data or state between multiple Svelte components without using props or context.

💼 Career

Understanding the Svelte store contract is important for building scalable and maintainable Svelte applications, a skill valued in frontend development roles.

1

Create the initial store object with a value

Create a variable called count and set it to 0.

Svelte

// Create a variable called count and set it to 0
// Your code here

Hint

Use let count = 0; to create the initial value.

2

Add a subscribers list to track listeners

Create a variable called subscribers and set it to an empty array [].

Svelte

let count = 0;
// Create an empty array called subscribers
// Your code here

Hint

Use const subscribers = []; to hold subscriber functions.

3

Implement the subscribe method for the store

Create a function called subscribe that takes a parameter run. Inside, add run to the subscribers array and immediately call run(count) to send the current value.

Svelte

let count = 0;
const subscribers = [];
// Create a function subscribe(run) that adds run to subscribers and calls run(count)
// Your code here

Hint

Remember to call run(count) right after adding the subscriber.

4

Add an update method to change the value and notify subscribers

Create a function called set that takes a parameter value. Inside, set count = value and then call each function in subscribers with the new count.

Svelte

let count = 0;
const subscribers = [];
function subscribe(run) {
  subscribers.push(run);
  run(count);
}
// Create a function set(value) that updates count and calls all subscribers
// Your code here

Hint

Use subscribers.forEach(run => run(count)) to notify all subscribers.

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