Auto-subscription with $ prefix in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Svelte component?

Consider this Svelte component using a writable store and the $ prefix for auto-subscription:

import { writable } from 'svelte/store';

const count = writable(0);

function increment() {
  count.update(n => n + 1);
}

<button on:click={increment}>Increment</button>
<p>Count: {$count}</p>

What will be displayed in the paragraph after clicking the button three times?

Svelte

import { writable } from 'svelte/store';

const count = writable(0);

function increment() {
  count.update(n => n + 1);
}

<button on:click={increment}>Increment</button>
<p>Count: {$count}</p>

A Count: undefined

B Count: 0

C Count: 3

D Count: NaN

💡 Hint

Remember that the $ prefix automatically subscribes to the store and updates the value reactively.

The $count syntax subscribes to the count store and updates the displayed value whenever the store changes. Clicking the button three times increments the count from 0 to 3, so the paragraph shows "Count: 3".

📝 Syntax

intermediate

Which option correctly uses the $ prefix to auto-subscribe a readable store?

Given a readable store user, which of the following Svelte code snippets correctly displays the user's name using auto-subscription?

Svelte

import { readable } from 'svelte/store';

const user = readable({ name: 'Alice' });

<p>User: ???</p>

A <p>User: {$user.name}</p>

B <p>User: {$user}.name</p>

C <p>User: {$user['name']}</p>

D <p>User: {$user().name}</p>

💡 Hint

The $ prefix unwraps the store value directly, so you can access properties on it normally.

Option A correctly uses {$user.name} to access the name property of the store's value. Option A tries to access .name outside the braces, which is invalid. Option A uses bracket notation inside the braces, which is invalid syntax. Option A treats $user as a function call, which is incorrect.

🔧 Debug

advanced

Why does this Svelte component throw an error when using $ prefix?

Examine this Svelte component code:

let count = 0;

function increment() {
  count += 1;
}

<button on:click={increment}>Increment</button>
<p>Count: {$count}</p>

Why does using {$count} cause an error here?

Svelte

let count = 0;

function increment() {
  count += 1;
}

<button on:click={increment}>Increment</button>
<p>Count: {$count}</p>

A Because $ prefix only works with Svelte stores, not plain variables

B Because count is not initialized before use

C Because the increment function is missing parentheses

D Because $count needs to be declared as a reactive statement

💡 Hint

Think about what the $ prefix does in Svelte.

The $ prefix in Svelte is a special syntax that auto-subscribes to stores. It does not work with normal variables like count. Since count is a plain number variable, using {$count} causes an error.

❓ state_output

advanced

What is the value of display after this code runs?

Given this Svelte store and reactive statement:

import { writable } from 'svelte/store';

const num = writable(5);

let display = '';

$: display = `Number is ${$num}`;

num.set(10);

What is the final value of display?

Svelte

import { writable } from 'svelte/store';

const num = writable(5);

let display = '';

$: display = `Number is ${$num}`;

num.set(10);

A Number is undefined

B Number is 10

C Number is $num

D Number is 5

💡 Hint

Reactive statements run whenever the store value changes.

The reactive statement $: display = `Number is ${$num}`; runs initially with $num as 5, then again after num.set(10) updates the store. So display ends as "Number is 10".

🧠 Conceptual

expert

Which statement about the $ prefix in Svelte is TRUE?

Choose the correct statement about the $ prefix auto-subscription feature in Svelte stores.

A The $ prefix only works with derived stores, not writable or readable stores

B The $ prefix can be used to auto-subscribe to any variable declared with let or const

C The $ prefix requires manual subscription and unsubscription in the component code

D Using the $ prefix automatically unsubscribes from the store when the component is destroyed

💡 Hint

Think about how Svelte manages store subscriptions with the $ prefix.

The $ prefix in Svelte automatically subscribes to the store and also handles unsubscribing when the component is destroyed, so you don't have to manage subscriptions manually. It works with writable, readable, and derived stores. It does not work with plain variables.