Store contract (subscribe method) in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the subscribe method in a Svelte store?

The subscribe method lets components listen for changes in the store's value. When the value updates, the subscriber function runs with the new value.

beginner

What does the subscribe method return in Svelte stores?

It returns an unsubscribe function. Calling this function stops the component from listening to store updates, helping avoid memory leaks.

intermediate

How does the subscribe method relate to the store contract in Svelte?

The store contract requires a subscribe method that accepts a callback and returns an unsubscribe function. This standard lets Svelte know how to react to store changes.

intermediate

Why is it important that the subscribe method immediately calls the subscriber with the current value?

It ensures the subscriber has the latest store value right away, so components can render correctly without waiting for a change.

advanced

Show a simple example of a custom Svelte store implementing the subscribe method.

Example:
<pre>function createStore() {
  let value = 0;
  const subscribers = new Set();
  return {
    subscribe(run) {
      run(value);
      subscribers.add(run);
      return () => subscribers.delete(run);
    },
    set(newValue) {
      value = newValue;
      subscribers.forEach(s => s(value));
    }
  };
}</pre>

What does the subscribe method in a Svelte store expect as its argument?

A An event name to listen for

B A new value to set in the store

C A boolean to enable or disable the store

D A callback function to run on value changes

The subscribe method takes a callback function that runs whenever the store's value changes.

What should the subscribe method return?

A An unsubscribe function

B The current store value

C A promise resolving to the value

D Nothing (void)

It returns an unsubscribe function to stop listening to store updates.

When a subscriber function is passed to subscribe, when is it called?

A Only when the store value changes after subscription

B Only once when the store is created

C Immediately with the current value and on every change

D Never, unless manually triggered

The subscriber is called immediately with the current value and then on every update.

Why is the unsubscribe function important in Svelte stores?

A To reset the store value

B To stop listening and avoid memory leaks

C To subscribe again later

D To change the store type

Unsubscribing stops updates and prevents memory leaks.

Which of these is part of the Svelte store contract?

A A <code>subscribe</code> method that returns an unsubscribe function

B A <code>subscribe</code> method that returns a promise

C A <code>subscribe</code> method that accepts no arguments

D A <code>subscribe</code> method that modifies the store value

The store contract requires subscribe to accept a callback and return an unsubscribe function.

Explain how the subscribe method works in the Svelte store contract and why it is important.

Think about how components listen and stop listening to store updates.

Describe how you would implement a simple custom store with a subscribe method in Svelte.

Focus on managing subscribers and notifying them.