Reactive declarations (let) in Svelte

Introduction

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

When you want to calculate a value based on other variables and update it automatically.

When you need to keep UI elements updated as data changes.

When you want to avoid writing manual event handlers to update related values.

When you want to simplify your code by letting Svelte handle dependencies.

When you want to create derived values that depend on user input or other reactive data.

Syntax

Svelte

let derived = expression;
$: derived = expression;

The $: label marks a reactive declaration.

Whenever variables inside the expression change, the reactive declaration runs again.

Examples

This creates a reactive variable doubled that updates when count changes.

Svelte

let count = 0;
$: doubled = count * 2;

The greeting updates automatically when name changes.

Svelte

let name = 'Alice';
$: greeting = `Hello, ${name}!`;

sum recalculates whenever a or b changes.

Svelte

let a = 5;
let b = 10;
$: sum = a + b;

Sample Program

This Svelte component shows a button with a count. When you click it, count increases. The doubled value updates automatically and shows below.

It uses a reactive declaration $: doubled = count * 2; to keep doubled in sync.

Svelte

<script>
  let count = 1;
  $: doubled = count * 2;

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

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

Important Notes

Reactive declarations run after the component initializes and whenever dependencies change.

You can use multiple reactive declarations in one component.

Use let to declare variables that will change; reactive declarations update them automatically.

Summary

Reactive declarations automatically update values when dependencies change.

Use $: before a statement to make it reactive.

This helps keep your UI and data in sync with less code.

Practice

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

easy

A. It imports external modules into the component.

B. It declares a variable as constant and unchangeable.

C. It comments out the following line of code.

D. It marks a statement as reactive, updating when dependencies change.

Solution

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

   The $: prefix is used to declare reactive statements that automatically update when their dependencies change.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated concepts: imports, constants, and comments, which are not related to $:.

3. Final Answer:

   It marks a statement as reactive, updating when dependencies change. -> Option D

4. Quick Check:

   Reactive declarations use $: = C [OK]

Hint: Remember: $: means reactive update in Svelte [OK]

Common Mistakes:

• Thinking $: declares constants
• Confusing $: with import syntax
• Assuming $: comments code

2. Which of the following is the correct syntax to create a reactive declaration for variable sum that adds a and b in Svelte?

easy

A. let sum = a + b;

B. sum := a + b;

C. $: sum = a + b;

D. const $sum = a + b;

Solution

1. Step 1: Identify reactive declaration syntax

   In Svelte, reactive declarations use $: before the statement to update automatically when dependencies change.

2. Step 2: Check each option

   let sum = a + b; is a normal variable assignment without reactivity. sum := a + b; uses invalid syntax. const $sum = a + b; incorrectly uses const and a dollar sign in the variable name.

3. Final Answer:

   $: sum = a + b; -> Option C

4. Quick Check:

   Reactive declaration = $: sum = a + b; [OK]

Hint: Reactive declarations start with $: followed by assignment [OK]

Common Mistakes:

• Omitting $: for reactive variables
• Using invalid assignment operators like ':='
• Trying to declare reactive constants with const

3. Given the code below, what will be the value of sum after a changes to 5?

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

// Later in code
a = 5;

medium

A. 8

B. 5

C. 3

D. NaN

Solution

1. Step 1: Understand reactive declaration behavior

   The reactive declaration $: sum = a + b; recalculates sum whenever a or b changes.

2. Step 2: Calculate new value after a changes

   Initially, a = 2 and b = 3, so sum = 5. When a changes to 5, sum updates to 5 + 3 = 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   sum updates reactively = 8 [OK]

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

Common Mistakes:

• Assuming sum stays old value after a changes
• Confusing sum with just a or b
• Expecting syntax error due to reassignment

4. Identify the error in this Svelte reactive declaration:

let x = 1;
$: y = x * 2
$: z = y + 3

medium

A. The code is correct and will work as expected.

B. Reactive declarations cannot depend on other reactive variables.

C. Variables must be declared with const inside reactive declarations.

D. Missing semicolon after y = x * 2 causes syntax error.

Solution

1. Step 1: Check syntax and reactive dependencies

   The code uses reactive declarations correctly. Semicolons are optional in JavaScript and Svelte allows reactive variables to depend on other reactive variables.

2. Step 2: Validate variable declarations

   Variables declared with let can be used reactively. const is not required inside reactive declarations.

3. Final Answer:

   The code is correct and will work as expected. -> Option A

4. Quick Check:

   Reactive chaining allowed = D [OK]

Hint: Reactive declarations can chain and omit semicolons safely [OK]

Common Mistakes:

• Thinking semicolons are mandatory
• Believing reactive vars can't depend on others
• Forcing const instead of let

5. You want to create a reactive variable status that shows "High" if score is above 80, "Medium" if between 50 and 80, and "Low" otherwise. Which reactive declaration correctly implements this?

hard

A. let status = if (score > 80) 'High' else if (score >= 50) 'Medium' else 'Low';

B. $: status = score > 80 ? 'High' : score >= 50 ? 'Medium' : 'Low';

C. $: if score > 80 then status = 'High' else status = 'Low';

D. $: status = score > 80 && 'High' || score >= 50 && 'Medium' || 'Low';

Solution

1. Step 1: Understand conditional expressions in reactive declarations

   Svelte reactive declarations use JavaScript syntax. Nested ternary operators correctly express multiple conditions.

2. Step 2: Evaluate each option's correctness

   $: status = score > 80 ? 'High' : score >= 50 ? 'Medium' : 'Low'; uses valid nested ternary syntax. let status = if (score > 80) 'High' else if (score >= 50) 'Medium' else 'Low'; uses invalid syntax (no if inside assignment). $: if score > 80 then status = 'High' else status = 'Low'; uses invalid syntax (no then keyword in JS). $: status = score > 80 && 'High' || score >= 50 && 'Medium' || 'Low'; uses logical operators that may cause incorrect evaluation.

3. Final Answer:

   $: status = score > 80 ? 'High' : score >= 50 ? 'Medium' : 'Low'; -> Option B

4. Quick Check:

   Nested ternary for reactive status = A [OK]

Hint: Use nested ternary operators for multi-condition reactive values [OK]

Common Mistakes:

• Using invalid if-else syntax inside reactive declarations
• Confusing logical operators with ternary conditions
• Omitting $: for reactive updates