If blocks ({#if}) in Svelte

Introduction

If blocks let you show or hide parts of your webpage based on conditions. It helps make your page change when things happen.

Show a message only when a user is logged in.

Display a loading spinner while data is loading.

Hide a button after it is clicked.

Show different content based on user choices.

Syntax

Svelte

{#if condition}
  <!-- content to show if true -->
{:else if anotherCondition}
  <!-- content if anotherCondition is true -->
{:else}
  <!-- content if all conditions are false -->
{/if}

Use {#if} to start the block and {/if} to end it.

You can add {:else if} and {:else} parts to handle multiple cases.

Examples

Shows a welcome message only if loggedIn is true.

Svelte

{#if loggedIn}
  <p>Welcome back!</p>
{/if}

Shows a pass or fail message depending on the score.

Svelte

{#if score >= 50}
  <p>You passed!</p>
{:else}
  <p>Try again.</p>
{/if}

Handles three states: loading, error, and loaded.

Svelte

{#if status === 'loading'}
  <p>Loading...</p>
{:else if status === 'error'}
  <p>Error occurred.</p>
{:else}
  <p>Data loaded.</p>
{/if}

Sample Program

This Svelte component shows a button to log in or out. When clicked, it toggles the loggedIn state. The message below changes based on whether the user is logged in or not.

The button uses aria-pressed for accessibility to indicate its state.

Svelte

<script>
  let loggedIn = false;
  function toggleLogin() {
    loggedIn = !loggedIn;
  }
</script>

<button on:click={toggleLogin} aria-pressed={loggedIn}>
  {loggedIn ? 'Logout' : 'Login'}
</button>

{#if loggedIn}
  <p>Welcome back, user!</p>
{:else}
  <p>Please log in to continue.</p>
{/if}

Important Notes

Always close your if blocks with {/if}.

Use aria-pressed or similar attributes on interactive elements for better accessibility.

Test your conditions carefully to avoid showing wrong content.

Summary

If blocks let you show or hide content based on conditions.

Use {#if}, {:else if}, and {:else} to handle different cases.

They help make your app interactive and responsive to user actions.

Practice

1. What does the {#if} block do in Svelte?

easy

A. It conditionally shows or hides content based on a true or false value.

B. It loops over a list of items to display them.

C. It styles elements with CSS inside the component.

D. It defines a reusable component.

Solution

1. Step 1: Understand the purpose of {#if}

   The {#if} block is used to check a condition and decide if some content should be shown or hidden.

2. Step 2: Compare with other options

   Looping is done with {#each}, styling is done with CSS, and reusable components are separate files or functions.

3. Final Answer:

   It conditionally shows or hides content based on a true or false value. -> Option A

4. Quick Check:

   Conditional display = C [OK]

Hint: If you want to show/hide, think {#if} block [OK]

Common Mistakes:

• Confusing {#if} with loops like {#each}
• Thinking {#if} styles elements
• Mixing {#if} with component creation

2. Which of the following is the correct syntax for an {#if} block in Svelte?

easy

A. {if condition} ... {endif}

B. <if condition> ... </if>

C. {#if condition} ... {/if}

D. {#if condition} ... {#end}

Solution

1. Step 1: Recall Svelte if block syntax

   Svelte uses {#if condition} ... {/if} to start and end an if block.

2. Step 2: Check other options for correctness

   Options A and C use incorrect syntax from other languages or HTML. {#if condition} ... {#end} uses a wrong closing tag.

3. Final Answer:

   {#if condition} ... {/if} -> Option C

4. Quick Check:

   Correct Svelte if syntax = B [OK]

Hint: Remember Svelte uses {/if} to close if blocks [OK]

Common Mistakes:

• Using HTML <if> tags instead of Svelte syntax
• Forgetting the # in {#if}
• Using wrong closing tags like {#end}

3. What will be rendered by this Svelte code?

{`\n{#if loggedIn}\n  
Welcome back!
\n{:else}\n  
Please log in.
\n{/if}`}

medium

A. Welcome back!

B. Please log in.

C. Nothing will be shown

D. Syntax error

Solution

1. Step 1: Check the value of loggedIn

   The variable loggedIn is set to false initially.

2. Step 2: Understand the if-else block behavior

   If loggedIn is true, it shows "Welcome back!"; else it shows "Please log in." Since loggedIn is false, the else block runs.

3. Final Answer:

   <p>Please log in.</p> -> Option B

4. Quick Check:

   loggedIn false shows else block = A [OK]

Hint: False condition shows {:else} content [OK]

Common Mistakes:

• Assuming false shows the if block content
• Ignoring the {:else} block
• Thinking no output if condition false

4. Identify the error in this Svelte code snippet:

{`\n{#if count > 3}\n  
Count is high
\n{:else if count > 1}\n  
Count is medium
\n{:else}\n  
Count is low
\n{/if}`}

medium

A. The syntax {:else if} is invalid in Svelte.

B. The variable count should be declared with const, not let.

C. The closing tag {/if} is missing.

D. There is no error; the code is correct.

Solution

1. Step 1: Check if {:else if} is valid in Svelte

   Svelte supports {:else if} syntax to chain conditions inside {#if} blocks.

2. Step 2: Verify variable declaration and closing tags

   Using let for count is correct for reactive variables. The {/if} closing tag is present.

3. Final Answer:

   There is no error; the code is correct. -> Option D

4. Quick Check:

   Valid else if syntax and proper closure = A [OK]

Hint: Svelte supports {:else if} inside {#if} blocks [OK]

Common Mistakes:

• Thinking {:else if} is invalid syntax
• Forgetting {/if} closing tag
• Confusing let and const for reactive vars

5. You want to display a message only if a user is logged in and has admin rights. Which Svelte {#if} block correctly checks both conditions?

hard

A. {#if loggedIn && isAdmin} Welcome, admin! {/if}

B. {#if loggedIn || isAdmin} Welcome, admin! {/if}

C. {#if loggedIn} {:if isAdmin} Welcome, admin! {/if} {/if}

D. {#if loggedIn | isAdmin} Welcome, admin! {/if}

Solution

1. Step 1: Understand the condition requirements

   The message should show only if both loggedIn and isAdmin are true, so we need a logical AND.

2. Step 2: Evaluate each option's logic

   {#if loggedIn | isAdmin}

   Welcome, admin!

   {/if} uses | bitwise OR which shows if either is true (wrong). {#if loggedIn || isAdmin}

   Welcome, admin!

   {/if} uses || logical OR which means either can be true (wrong). {#if loggedIn} {:if isAdmin}

   Welcome, admin!

   {/if} {/if} uses invalid {:if} syntax. {#if loggedIn && isAdmin}

   Welcome, admin!

   {/if} uses && logical AND which requires both true (correct).

3. Final Answer:

   {#if loggedIn && isAdmin} <p>Welcome, admin!</p> {/if} -> Option A

4. Quick Check:

   Use && for both true conditions [OK]

Hint: Use && to require both conditions true in {#if} [OK]

Common Mistakes:

• Using || instead of && for both conditions
• Using bitwise | instead of logical &&
• Using invalid {:if} syntax