Reactive statements ($:) in Svelte

Introduction

Reactive statements let your code automatically update when data changes. This helps keep your app in sync without extra work.

You want to update a value whenever another value changes.

You need to run some code automatically after a variable updates.

You want to keep your UI and data connected easily.

You want to avoid writing manual event handlers for simple updates.

Syntax

Svelte

$: statement_or_assignment;

The $: prefix marks a reactive statement.

It runs whenever any variable it uses changes.

Examples

This sets doubled to twice count whenever count changes.

Svelte

$: doubled = count * 2;

This logs a message every time count updates.

Svelte

$: console.log('Count changed:', count);

You can use a block to run multiple statements reactively.

Svelte

$: {
  total = price * quantity;
  console.log('Total updated:', total);
}

Sample Program

This Svelte component shows a button that increases count. The doubled value updates automatically using a reactive statement. The UI updates to show both values.

Svelte

<script>
  let count = 0;
  $: doubled = count * 2;
</script>

<button on:click={() => count++} aria-label="Increase count">Increase</button>
<p>Count: {count}</p>
<p>Doubled: {doubled}</p>

Important Notes

Reactive statements run after the component updates the variables they depend on.

You can use reactive statements for calculations, side effects, or logging.

Be careful to avoid infinite loops by not updating a variable inside its own reactive statement without conditions.

Summary

Reactive statements automatically run when their data changes.

Use $: before a statement or assignment to make it reactive.

This helps keep your UI and data in sync easily.

Practice

1. What does the $: symbol do in a Svelte component?

easy

A. It imports a module asynchronously.

B. It marks a statement as reactive, so it runs when its dependencies change.

C. It declares a variable as a prop to the component.

D. It defines a CSS style block scoped to the component.

Solution

1. Step 1: Understand the role of $: in Svelte

   The $: prefix is used to mark statements as reactive, meaning they automatically re-run when any variable they use changes.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated Svelte features: async imports, props, and scoped styles, which do not use $:.

3. Final Answer:

   It marks a statement as reactive, so it runs when its dependencies change. -> Option B

4. Quick Check:

   Reactive statement = runs on data change [OK]

Hint: Remember $: means 'run this when data changes' [OK]

Common Mistakes:

• Confusing $: with props declaration
• Thinking $: imports modules
• Assuming $: styles CSS

2. Which of the following is the correct syntax to create a reactive statement in Svelte?

easy

A. let $: total = a + b;

B. const $total = a + b;

C. reactive total = a + b;

D. $: total = a + b;

Solution

1. Step 1: Identify correct reactive statement syntax

   In Svelte, reactive statements start with $: followed by an assignment or expression, e.g. $: total = a + b;.

2. Step 2: Eliminate incorrect options

   let $: total = a + b; incorrectly places $: after let. reactive total = a + b; uses a non-existent keyword reactive. const $total = a + b; uses $ in variable name incorrectly.

3. Final Answer:

   $: total = a + b; -> Option D

4. Quick Check:

   Reactive statement syntax = $: expression [OK]

Hint: Start reactive lines with $: and no let/const [OK]

Common Mistakes:

• Using let or const with $: reactive statements
• Writing $: inside variable names
• Using non-existent keywords like reactive

3. Consider this Svelte code:

let a = 2;
let b = 3;
$: sum = a + b;

// Later in code
b = 5;

What is the value of sum after b changes?

medium

A. 7

B. 5

C. 8

D. 3

Solution

1. Step 1: Understand reactive statement behavior

   The reactive statement $: sum = a + b; runs whenever a or b changes. Initially, a=2 and b=3, so sum=5.

2. Step 2: Update after changing b

   When b changes to 5, the reactive statement re-runs, updating sum = 2 + 5 = 7. But note the options carefully: 8 is 8, which is 2 + 6, not correct. 7 is 7, which matches calculation.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   sum updates automatically = 7 [OK]

Hint: Reactive sums update immediately when inputs change [OK]

Common Mistakes:

• Not updating sum after b changes
• Adding wrong numbers
• Confusing initial and updated values

4. Identify the error in this Svelte reactive statement:

let count = 0;
$: count = count + 1;

medium

A. Reactive statements cannot use let variables.

B. Syntax error: missing semicolon after count declaration.

C. This causes an infinite loop because count updates itself reactively.

D. No error; this code increments count once.

Solution

1. Step 1: Analyze reactive statement effect

   The statement $: count = count + 1; runs whenever count changes. But it changes count itself, causing it to run again immediately, creating an infinite loop.

2. Step 2: Check other options

   Syntax error: missing semicolon after count declaration. is incorrect because semicolons are optional in JS. Reactive statements cannot use let variables. is false; reactive statements can use let variables. No error; this code increments count once. is wrong because the increment repeats endlessly.

3. Final Answer:

   This causes an infinite loop because count updates itself reactively. -> Option C

4. Quick Check:

   Reactive self-update causes infinite loop [OK]

Hint: Avoid reactive statements that update their own dependencies [OK]

Common Mistakes:

• Ignoring infinite loops from self-updating reactive statements
• Thinking semicolons cause errors
• Believing reactive statements can't use let variables

5. You want to create a reactive statement that updates discountedPrice only when price or discount change, but discount can sometimes be zero or null. Which reactive statement correctly handles this so discountedPrice is price when discount is falsy?

hard

A. $: discountedPrice = discount ? price - (price * discount) : price;

B. $: discountedPrice = price - (price * discount);

C. $: discountedPrice = price - (price * (discount || 1));

D. $: discountedPrice = price * discount || price;

Solution

1. Step 1: Understand the problem with falsy discount

   If discount is zero or null, we want discountedPrice to equal price (no discount). A simple calculation without check will give wrong results.

2. Step 2: Evaluate each option

   $: discountedPrice = price - (price * discount); always subtracts price * discount, so zero discount works but null may cause NaN. $: discountedPrice = discount ? price - (price * discount) : price; uses a conditional to check if discount is truthy, else returns price, correctly handling zero and null. $: discountedPrice = price - (price * (discount || 1)); uses discount || 1, which makes discount 1 if falsy, causing full price discount (wrong). $: discountedPrice = price * discount || price; multiplies price by discount or price, which gives wrong results when discount is zero.

3. Final Answer:

   $: discountedPrice = discount ? price - (price * discount) : price; -> Option A

4. Quick Check:

   Use conditional to handle falsy discount [OK]

Hint: Use conditional ?: to handle zero or null discount safely [OK]

Common Mistakes:

• Ignoring falsy discount causing wrong calculations
• Using || 1 which changes discount incorrectly
• Assuming zero discount subtracts correctly without check