Await blocks ({#await}) in Svelte - Mini Project: Build & Apply

Using Await Blocks in Svelte

📖 Scenario: You are building a simple Svelte app that fetches a user's profile data from a server. The data takes some time to load, so you want to show a loading message while waiting, display the profile when ready, and show an error message if something goes wrong.

🎯 Goal: Create a Svelte component that uses the {#await} block to handle asynchronous data fetching. The component should show a loading message while waiting, display the user name when the data is loaded, and show an error message if the fetch fails.

📋 What You'll Learn

Create a promise that simulates fetching user data

Create a variable to hold the promise

Use the {#await} block with then and catch sections

Display appropriate messages for loading, success, and error states

💡 Why This Matters

🌍 Real World

Await blocks are useful in real apps to show loading states and errors when fetching data from servers.

💼 Career

Understanding how to handle asynchronous data and show feedback to users is essential for frontend developers working with modern frameworks like Svelte.

1

Create a promise to fetch user data

Create a variable called userPromise that is a new Promise. The promise should resolve after 2 seconds with an object { name: 'Alice' }.

Svelte

# Create a variable userPromise that resolves with { name: 'Alice' } after 2 seconds
# Your code here

Hint

Use new Promise and setTimeout to simulate a delayed fetch.

2

Add a loading message variable

Create a variable called loadingMessage and set it to the string 'Loading user data...'.

Svelte

let userPromise = new Promise((resolve) => {
    setTimeout(() => {
        resolve({ name: 'Alice' });
    }, 2000);
});

# Create a variable loadingMessage with the text 'Loading user data...'
# Your code here

Hint

Just create a simple string variable for the loading text.

3

Use the {#await} block to handle the promise

In the Svelte component markup, use the {#await userPromise} block. Inside it, show {loadingMessage} while waiting. Use then user to display <p>User name: {user.name}</p>. Use catch error to display <p>Error loading user data</p>.

Svelte

let userPromise = new Promise((resolve) => {
    setTimeout(() => {
        resolve({ name: 'Alice' });
    }, 2000);
});

let loadingMessage = 'Loading user data...';






# Your code here

Hint

Use the Svelte {#await} block with then and catch sections to handle the promise states.

4

Add error simulation and final markup

Modify the userPromise to sometimes reject with an error message 'Failed to fetch' after 2 seconds. Keep the rest of the code the same. This simulates a fetch failure. Ensure the {#await} block handles this error and shows the error message.

Svelte

let loadingMessage = 'Loading user data...';

{#await userPromise}
    {loadingMessage}
{:then user}
    User name: {user.name}
{:catch error}
    Error loading user data
{/await}

# Modify userPromise to sometimes reject with 'Failed to fetch'
# Your code here

Hint

Use reject inside the promise to simulate an error. The {#await} block will show the error message.

Practice

1. What is the main purpose of the {#await} block in Svelte?

easy

A. To handle asynchronous data by showing loading, success, and error states

B. To create loops for rendering lists

C. To define reactive variables

D. To style components conditionally

Solution

1. Step 1: Understand the role of {#await} in Svelte

   The {#await} block is designed to handle promises and asynchronous data by showing different UI states while waiting, on success, or on error.

2. Step 2: Compare with other options

   Creating loops is done with {#each}, defining reactive variables with let or $:, and styling conditionally with class: or style:, which are not the purpose of {#await}.

3. Final Answer:

   To handle asynchronous data by showing loading, success, and error states -> Option A

4. Quick Check:

   Await blocks = handle async states [OK]

Hint: Await blocks manage async loading and errors [OK]

Common Mistakes:

• Confusing await blocks with loops
• Thinking await blocks define reactive variables
• Assuming await blocks are for styling

2. Which of the following is the correct syntax to start an await block in Svelte for a promise named dataPromise?

easy

A. {#await: dataPromise} ... {/await}

B. {#await (dataPromise)} ... {/await}

C. {#await await dataPromise} ... {/await}

D. {#await dataPromise} ... {/await}

Solution

1. Step 1: Recall the basic syntax of await blocks

   The correct syntax is {#await promise} followed by the block content and closed with {/await}.

2. Step 2: Analyze each option

   {#await dataPromise} ... {/await} matches the correct syntax. {#await (dataPromise)} ... {/await} adds unnecessary parentheses. {#await await dataPromise} ... {/await} repeats 'await' incorrectly. {#await: dataPromise} ... {/await} uses a colon which is invalid.

3. Final Answer:

   {#await dataPromise} ... {/await} -> Option D

4. Quick Check:

   Correct await syntax = {#await dataPromise} ... {/await} [OK]

Hint: Use {#await promise} without extra symbols [OK]

Common Mistakes:

• Adding parentheses around the promise
• Repeating the 'await' keyword inside the block
• Using colons after #await

3. Given the following Svelte code snippet:

{#await fetchData()}
  
Loading...

:then data
  
{data.message}

:catch error
  
Error: {error.message}

{/await}

What will be displayed if fetchData() returns a promise that resolves to { message: 'Hello!' }?

medium

A. Loading...

B. Error: ...

C. Hello!

D. Nothing will be displayed

Solution

1. Step 1: Understand the promise resolution

   The promise returned by fetchData() resolves successfully with an object containing { message: 'Hello!' }.

2. Step 2: Determine which await block section runs

   Since the promise resolves, the :then block runs, displaying {data.message} which is 'Hello!'.

3. Final Answer:

   <p>Hello!</p> -> Option C

4. Quick Check:

   Resolved promise shows then block [OK]

Hint: Resolved promise shows :then content [OK]

Common Mistakes:

• Thinking loading shows after resolution
• Confusing :catch with :then
• Expecting no output on success

4. Identify the error in this Svelte await block:

{#await loadData()}
  
Loading...

:then result
  
{result}

:catch
  
Error occurred

{/await}

medium

A. Incorrect use of :then keyword

B. Missing variable name after :catch

C. No closing {/await} tag

D. Promises cannot be used in await blocks

Solution

1. Step 1: Review the :catch syntax in await blocks

   The :catch block must include a variable name to capture the error, like :catch error.

2. Step 2: Check the given code

   The code uses :catch without a variable name, which is a syntax error.

3. Final Answer:

   Missing variable name after :catch -> Option B

4. Quick Check:

   :catch needs error variable [OK]

Hint: Always name error after :catch [OK]

Common Mistakes:

• Omitting error variable in :catch
• Forgetting {/await} closing tag
• Misusing :then keyword

5. You want to show a loading spinner while waiting for userData promise, display the user's name on success, and show a retry button on error. Which Svelte await block correctly implements this behavior?

hard

A. {#await userData} :then user {user.name}:catch error Retry{/await}

B. {#await userData} Loading...:then user {user.name}{/await}

C. {#await userData} :catch error Retry{/await}

D. {#await userData} :then user {user.name}:catch Retry{/await}

Solution

1. Step 1: Verify loading, success, and error states

   {#await userData}
   
   :then user
   

   {user.name}

   
   :catch error
   Retry
   {/await} shows a spinner during loading, displays user name on success with :then user, and shows a retry button with error variable error in :catch.

2. Step 2: Check other options for missing parts

   {#await userData}
   

   Loading...

   
   :then user
   

   {user.name}

   
   {/await} lacks error handling. {#await userData}
   
   :catch error
   Retry
   {/await} lacks success handling. {#await userData}
   
   :then user
   

   {user.name}

   
   :catch
   Retry
   {/await} misses error variable after :catch.

3. Final Answer:

   {#await userData}
   
   :then user
   

   {user.name}

   
   :catch error
   Retry
   {/await} -> Option A

4. Quick Check:

   All states with variables = {#await userData}
   
   :then user
   

   {user.name}

   
   :catch error
   Retry
   {/await} [OK]

Hint: Include loading, then, catch with variables [OK]

Common Mistakes:

• Omitting :catch block or error variable
• Missing :then block for success
• Not showing loading state properly