Store contract (subscribe method) in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to a Svelte store and log its value.

Svelte

myStore.[1](value => console.log(value));

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

Asubscribe

Bset

Cupdate

Ddispatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' or 'update' instead of 'subscribe' to listen to store changes.

The subscribe method is used to listen to changes in a Svelte store.

2fill in blank

medium

Complete the code to unsubscribe from a Svelte store subscription.

Svelte

const unsubscribe = myStore.subscribe(() => {});
unsubscribe[1];

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

A;

B{}

C[]

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add parentheses when calling the unsubscribe function.

The unsubscribe function returned by subscribe must be called as a function to stop listening.

3fill in blank

hard

Fix the error in the subscribe callback parameter to correctly receive the store value.

Svelte

myStore.subscribe([1] => {
  console.log([1]);
});

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

Avalue

B()

Cstore

Devent

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the callback parameter empty or using incorrect parameter names.

The callback receives the current store value as its first argument, commonly named value.

4fill in blank

hard

Fill both blanks to create a custom store with a subscribe method that calls the subscriber immediately.

Svelte

function createStore() {
  return {
    subscribe: function([1]) {
      [2]('hello');
      return () => {};
    }
  };
}

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

Arun

Bsubscriber

Cnotify

Dcallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and the function call.

The subscribe method takes a subscriber function and calls it immediately with the current value.

5fill in blank

hard

Fill all three blanks to implement a store with subscribe, set, and update methods following the Svelte store contract.

Svelte

function createCounter() {
  let count = 0;
  const subscribers = new Set();

  function notify() {
    subscribers.forEach([1] => [2](count));
  }

  return {
    subscribe([3]) {
      subscribers.add([3]);
      [3](count);
      return () => subscribers.delete([3]);
    },
    set(value) {
      count = value;
      notify();
    },
    update(fn) {
      count = fn(count);
      notify();
    }
  };
}

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

Asubscriber

Bcallback

Cfn

Dlistener

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the subscriber parameter in different places.

Not calling the subscriber immediately after adding it.

The subscribe method adds a subscriber function to the set and calls it immediately with the current value. The notify function calls each subscriber with the updated count.

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