Await blocks ({#await}) in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The {#await} block lets you handle promises directly in your Svelte templates. It shows different content while waiting, when resolved, or if an error occurs.

beginner

How do you show a loading message while waiting for a promise in a Svelte {#await} block?

Use the pending part of the block. The content before :then runs while waiting (pending). For example:
{#await promise} Loading... {/await}

intermediate

How can you handle errors in a Svelte {#await} block?

Use the :catch block inside {#await}. It shows content if the promise rejects. Example:
{#await promise}
Loading...
:then data
Show data
:catch error
Show error message
{/await}

intermediate

What happens if you omit the :then block in a Svelte {#await} block?

If you omit :then, the block only shows the pending or catch content. You won't see the resolved data because there's no place to display it.

beginner

Can you use multiple {#await} blocks in one Svelte component?

Yes, you can use as many {#await} blocks as you want. Each handles its own promise independently, letting you show loading, success, or error states for each async task.

What part of the {#await} block runs while the promise is still loading?

A The content before <code>:then</code>

B The content inside <code>:then</code>

C The content inside <code>:catch</code>

D The content after <code>{/await}</code>

The content before :then runs while the promise is pending (loading).

How do you access the resolved value of a promise inside a {#await} block?

A Inside the <code>:then</code> block with a variable

B Inside the <code>:catch</code> block

C Before the <code>{#await}</code> block

D You cannot access it

The resolved value is passed as a variable to the :then block.

What does the :catch block do in a {#await} block?

A Shows content after the promise resolves

B Shows content while the promise is loading

C Cancels the promise

D Shows content if the promise rejects with an error

The :catch block handles errors if the promise fails.

Is it possible to omit the :catch block in a {#await} block?

A No, it is required

B Yes, but errors won’t be handled in the UI

C Yes, and errors will be automatically handled

D No, it causes a syntax error

You can omit :catch, but then errors won’t show any UI feedback.

Can you nest {#await} blocks inside each other in Svelte?

A No, nesting is not allowed

B Only if the promises are related

C Yes, to handle multiple async steps

D Only inside <code>:catch</code> blocks

You can nest {#await} blocks to handle multiple async operations step-by-step.

Explain how to use the {#await} block in Svelte to show loading, success, and error states for a promise.

Think about the three parts of the block: waiting, success, and failure.

Describe what happens if you omit the :then block in a {#await} block and why you might want to include it.

Consider how the UI changes when the promise finishes.

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