Await blocks ({#await}) in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show a loading message while waiting for the promise.

Svelte

<script>
  let dataPromise = fetch('/api/data').then(res => res.json());
</script>

{#await [1]
  <p>Loading...</p>
{/await}

Drag options to blanks, or click blank then click option'

AfetchData()

Bdata

CdataPromise

DpromiseData

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-promise variable inside the await block.

Forgetting to use the promise variable name.

The {#await} block waits for the promise dataPromise to resolve or reject.

2fill in blank

medium

Complete the code to display the resolved data inside the await block.

Svelte

{#await dataPromise}
  <p>Loading...</p>
{:then [1]
  <p>{user.name}</p>
{/await}

Drag options to blanks, or click blank then click option'

Auser

Bresponse

Cresult

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than the one declared after :then.

Not declaring a variable after :then.

The :then block receives the resolved value, here named user.

3fill in blank

hard

Fix the error in the await block to handle errors properly.

Svelte

{#await dataPromise}
  <p>Loading...</p>
{:then user}
  <p>{user.name}</p>
{:catch [1]
  <p>Error: {error.message}</p>
{/await}

Drag options to blanks, or click blank then click option'

Ae

Berr

Cexception

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than the one inside the error message.

Not declaring a variable after :catch.

The variable after :catch must match the one used inside the block, here error.

4fill in blank

hard

Fill both blanks to show a loading message and handle the resolved data.

Svelte

{#await [1]
  <p>Loading data...</p>
{:then [2]
  <p>{info.title}</p>
{/await}

Drag options to blanks, or click blank then click option'

AfetchInfo()

BdataPromise

Cinfo

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the promise variable and the resolved data variable.

Using a function call instead of a promise variable in the await block.

The {#await} block waits for dataPromise. The resolved data is named info.

5fill in blank

hard

Fill all three blanks to handle loading, success, and error states in the await block.

Svelte

{#await [1]
  <p>Loading...</p>
{:then [2]
  <p>{data.message}</p>
{:catch [3]
  <p>Error: {err.message}</p>
{/await}

Drag options to blanks, or click blank then click option'

AfetchMessage()

Bdata

Cerr

DmessagePromise

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names between the catch block and inside it.

Not matching the promise variable name in the await block.

The promise is messagePromise. The resolved data is data. The error variable is err.

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