Auto-subscription with $ prefix in Svelte

Introduction

Auto-subscription with the $ prefix lets you easily use reactive store values in your Svelte components without manually subscribing and unsubscribing.

When you want to display a value from a Svelte store directly in your component.

When you want your component to automatically update when the store value changes.

When you want simpler code without writing subscription logic.

When you want to reactively use store values inside your component's markup or script.

Syntax

Svelte

let value = $storeName;

The $ prefix unwraps the current value of the store.

It automatically subscribes and unsubscribes when the component mounts and unmounts.

Examples

This example shows how to get the current value of a writable store count using $count.

Svelte

import { writable } from 'svelte/store';

const count = writable(0);

// In a Svelte component script
let currentCount = $count;

Here, $time automatically updates currentTime whenever the time store changes.

Svelte

<script>
  import { readable } from 'svelte/store';
  const time = readable(new Date());
  
  // Use auto-subscription
  let currentTime = $time;
</script>

You can also use $name directly in the markup to show the current store value reactively.

Svelte

<script>
  import { writable } from 'svelte/store';
  const name = writable('Alice');
</script>

<p>Hello, {$name}!</p>

Sample Program

This Svelte component uses a writable store count. The $count syntax auto-subscribes to the store and shows the current count. Clicking the button updates the store, and the displayed count updates automatically.

Svelte

<script>
  import { writable } from 'svelte/store';

  const count = writable(0);

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

<button on:click={increment} aria-label="Increment count">Increment</button>
<p>Count is: {$count}</p>

Important Notes

The $ prefix only works inside Svelte components.

It works with any Svelte store type: writable, readable, or derived.

Using $store is cleaner and safer than manual subscriptions.

Summary

The $ prefix unwraps store values automatically in Svelte components.

It keeps your component reactive and updates the UI when the store changes.

It simplifies your code by removing the need for manual subscribe/unsubscribe calls.

Practice

1. What does the $ prefix do when used with a Svelte store inside a component?

easy

A. It automatically subscribes to the store and updates the value reactively.

B. It creates a new store with the same value.

C. It unsubscribes from the store to stop updates.

D. It converts the store value to a string.

Solution

1. Step 1: Understand the role of the $ prefix

   The $ prefix in Svelte is used to auto-subscribe to a store and unwrap its current value.

2. Step 2: Recognize reactive updates

   Using $store makes the component update automatically when the store value changes, without manual subscription.

3. Final Answer:

   It automatically subscribes to the store and updates the value reactively. -> Option A

4. Quick Check:

   Auto-subscription = It automatically subscribes to the store and updates the value reactively. [OK]

Hint: Remember: $ means auto-subscribe and unwrap store value [OK]

Common Mistakes:

• Thinking $ creates a new store
• Believing $ unsubscribes from the store
• Assuming $ converts value to string

2. Which of the following is the correct syntax to use the $ prefix with a Svelte store named count inside a component?

easy

A. let value = count.$;

B. let value = count$;

C. let value = $count;

D. let value = $count();

Solution

1. Step 1: Identify correct $ prefix usage

   The $ prefix is placed directly before the store variable name to unwrap its value, like $count.

2. Step 2: Check syntax correctness

   let value = $count; uses let value = $count; which is valid syntax to get the current store value reactively.

3. Final Answer:

   let value = $count; -> Option C

4. Quick Check:

   Correct $ prefix syntax = let value = $count; [OK]

Hint: Place $ directly before store name, no parentheses [OK]

Common Mistakes:

• Putting $ after the store name
• Using $ as a method call
• Adding dot before $

3. Given the following Svelte code snippet, what will be displayed when the button is clicked twice?

  
   count.update(n => n + 1)}>Increment
  
Count: {$count}

medium

A. Count: undefined

B. Count: 1

C. Count: 0

D. Count: 2

Solution

1. Step 1: Understand the initial state and update

   The store count starts at 0. Each button click increases it by 1 using count.update(n => n + 1).

2. Step 2: Calculate value after two clicks

   After two clicks, the value increments twice: 0 + 1 + 1 = 2. The {$count} auto-subscribes and shows the current value.

3. Final Answer:

   Count: 2 -> Option D

4. Quick Check:

   2 increments = Count: 2 [OK]

Hint: Each click adds 1; $count shows current value [OK]

Common Mistakes:

• Forgetting the store updates on click
• Thinking $count is static
• Assuming initial value stays

4. What is the error in the following Svelte code using the $ prefix?

  
  
Hello, {currentName}!
   name.set('Bob')}>Change Name

medium

A. The store name cannot be updated with set.

B. The variable currentName should use $name to auto-subscribe.

C. The on:click event is missing parentheses.

D. The import statement is incorrect.

Solution

1. Step 1: Identify how to access store value reactively

   The variable currentName is assigned the store itself, not its value. To get the current value reactively, it should be let currentName = $name;.

2. Step 2: Understand the effect on UI

   Without $, currentName is the store object, so the UI shows it incorrectly and does not update on changes.

3. Final Answer:

   The variable currentName should use $name to auto-subscribe. -> Option B

4. Quick Check:

   Use $ to unwrap store value = The variable currentName should use $name to auto-subscribe. [OK]

Hint: Use $ prefix to unwrap store value for reactive variables [OK]

Common Mistakes:

• Assigning store object instead of its value
• Forgetting $ prefix for reactive updates
• Misunderstanding store.set usage

5. You have two writable stores in Svelte: countA and countB. You want to display their sum reactively in a component. Which is the correct way to do this using the $ prefix?

  
  
  
Sum: ???

hard

A. <p>Sum: {$countA + $countB}</p>

B. <p>Sum: {countA + countB}</p>

C. <p>Sum: {$countA + countB}</p>

D. <p>Sum: {countA.get() + countB.get()}</p>

Solution

1. Step 1: Understand how to unwrap multiple stores

   Using $countA and $countB unwraps their current values reactively.

2. Step 2: Combine unwrapped values in expression

   Adding $countA + $countB inside curly braces calculates the sum reactively and updates UI on changes.

3. Final Answer:

   <p>Sum: {$countA + $countB}</p> -> Option A

4. Quick Check:

   Use $ on both stores to sum reactively = <p>Sum: {$countA + $countB}</p> [OK]

Hint: Use $ on each store to unwrap before adding [OK]

Common Mistakes:

• Adding store objects instead of values
• Mixing $ prefix on only one store
• Using .get() which is not a Svelte store method