Text interpolation with {} in Svelte

Introduction

Text interpolation lets you show dynamic values inside your HTML easily. It helps your webpage update automatically when data changes.

Showing a user's name on a welcome message.

Displaying the current date or time that updates.

Showing the result of a calculation or variable.

Updating text based on user input instantly.

Displaying data fetched from an API.

Syntax

Svelte

<p>{variable}</p>

Use curly braces {} inside your HTML to insert JavaScript expressions or variables.

Svelte updates the displayed text automatically when the variable changes.

Examples

Shows the value of the variable name inside a paragraph.

Svelte

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

Calculates and shows the sum of a and b directly in the text.

Svelte

<p>The sum is {a + b}</p>

Displays the current value of count, which can change over time.

Svelte

<p>{count} clicks</p>

Sample Program

This Svelte component shows a greeting with a name and a button. Each time you click the button, the count variable increases, and the displayed text updates automatically.

Svelte

<script>
  let name = 'Alice';
  let count = 0;
  function increment() {
    count += 1;
  }
</script>

<main>
  <h1>Hello, {name}!</h1>
  <button on:click={increment}>Click me</button>
  <p>You clicked {count} times.</p>
</main>

Important Notes

Text interpolation with {} works inside HTML elements and in attribute values like class or id.

You can put any JavaScript expression inside the curly braces, but keep it simple for readability.

Summary

Use {} to insert variables or expressions inside your HTML.

Svelte updates the displayed text automatically when data changes.

It makes your UI dynamic and interactive with very little code.

Practice

1. In Svelte, how do you display the value of a variable name inside your HTML?

easy

A. Use {name} inside the HTML to show the variable's value.

B. Use <name> tags around the variable.

C. Use $name directly in the HTML.

D. Use $(name) inside the HTML.

Solution

1. Step 1: Understand Svelte's text interpolation syntax

   Svelte uses curly braces {} to insert variables or expressions inside HTML. To display the value of name, write {name} inside the HTML.

2. Final Answer:

   Use {name} inside the HTML to show the variable's value. -> Option A

3. Quick Check:

   Text interpolation = {name} [OK]

Hint: Remember: Curly braces {} show variables in Svelte [OK]

Common Mistakes:

• Using $name instead of {name}
• Trying to use HTML tags like <name>
• Using parentheses like $(name)

2. Which of the following is the correct syntax to show the result of 2 + 3 inside a Svelte component?

easy

A. $(2 + 3)

B. {{2 + 3}}

C. {2 + 3}

D. <2 + 3>

Solution

1. Step 1: Recall Svelte's expression interpolation

   Svelte allows expressions inside curly braces {} to be evaluated and shown. Only {2 + 3} correctly uses curly braces for expression evaluation; others are invalid.

2. Final Answer:

   {2 + 3} -> Option C

3. Quick Check:

   Expression in {} = {2 + 3} [OK]

Hint: Use curly braces {} for expressions, not $ or double braces [OK]

Common Mistakes:

• Using $(2 + 3) which is invalid in Svelte
• Using double curly braces {{}} like in other frameworks
• Using angle brackets around expressions

3. What will be displayed when this Svelte code runs?

<script>
  let count = 5;
</script>
<p>Count is: {count * 2}</p>

medium

A. Count is: 10

B. Count is: count * 2

C. Count is: 5

D. Count is: {count * 2}

Solution

1. Step 1: Evaluate the expression in the component

   The variable count is 5, and count * 2 equals 10, so "Count is: 10" is displayed.

2. Final Answer:

   Count is: 10 -> Option A

3. Quick Check:

   count * 2 = 10 [OK]

Hint: Calculate expressions inside {} before display [OK]

Common Mistakes:

• Showing the expression text instead of its value
• Not multiplying and showing just the variable
• Leaving braces visible in output

4. Identify the error in this Svelte code snippet:

<script>
  let message = 'Hello';
</script>
<p>{message</p>

medium

A. Use parentheses instead of braces (message)

B. Variable 'message' is not declared

C. Curly braces should be double like {{message}}

D. Missing closing brace '}' after {message}

Solution

1. Step 1: Check the interpolation syntax

   The code opens { but lacks closing } after message. Proper syntax is {message}.

2. Final Answer:

   Missing closing brace '}' after {message} -> Option D

3. Quick Check:

   Interpolation needs matching {} [OK]

Hint: Always close your curly braces {} in interpolation [OK]

Common Mistakes:

• Forgetting the closing brace '}'
• Using double braces {{}} which is invalid in Svelte
• Not declaring the variable before use

5. You want to display a user's full name by combining firstName and lastName inside a Svelte component. Which code correctly shows "Full Name: John Doe" if firstName = 'John' and lastName = 'Doe'?

hard

A. <script>
let firstName = 'John';
let lastName = 'Doe';
</script>
<p>Full Name: {firstName, ' ', lastName}</p>

B. <script>
let firstName = 'John';
let lastName = 'Doe';
</script>
<p>Full Name: {firstName + ' ' + lastName}</p>

C. <script>
let firstName = 'John';
let lastName = 'Doe';
</script>
<p>Full Name: {firstName & lastName}</p>

D. <script>
let firstName = 'John';
let lastName = 'Doe';
</script>
<p>Full Name: {firstName.concat(lastName)}</p>

Solution

1. Step 1: Identify correct string concatenation

   Svelte evaluates JS expressions in {}. Use +: {firstName + ' ' + lastName} gives "John Doe". Commas invoke comma operator (returns lastName), & is bitwise, concat(lastName) gives "JohnDoe" (no space).

2. Final Answer:

   Full Name: {firstName + ' ' + lastName} -> Option B

3. Quick Check:

   Use + to join strings inside {} [OK]

Hint: Use + to join strings inside {} for full text [OK]

Common Mistakes:

• Using commas instead of + for concatenation
• Using & which is a bitwise operator
• Forgetting space between names