Await blocks ({#await}) in Svelte - Step-by-Step Execution

Concept Flow - Await blocks ({#await})

Start: Promise created

↓

Render {#await} block

↓

Check Promise state

↓

Pending

↓

Show pending UI

↓

Update DOM

The await block watches a promise and shows different UI for pending, success, or error states.

Execution Sample

Svelte

  <script>
    let promise = fetchData();
  </script>

  {#await promise}
    <p>Loading...</p>
  {:then data}
    <p>Data: {data}</p>
  {:catch error}
    <p>Error: {error.message}</p>
  {/await}

This code shows loading text while waiting, then data or error after promise settles.

Execution Table

Step	Promise State	Block Rendered	UI Shown	Notes
1	Pending	Pending block	<p>Loading...</p>	Promise just started, waiting for result
2	Fulfilled	Then block	<p>Data: {data}</p>	Promise resolved successfully with data
3	Rejected	Catch block	<p>Error: {error.message}</p>	Promise rejected with error
4	N/A	End	UI updated accordingly	Rendering finished based on promise state

💡 Promise settles (fulfilled or rejected), await block updates UI and stops pending state

Variable Tracker

Variable	Start	After Pending	After Fulfilled	After Rejected
promise	fetchData() call	pending	resolved with data	rejected with error
data	undefined	undefined	actual data value	undefined
error	undefined	undefined	undefined	error object

Key Moments - 3 Insights

Why does the pending block show before the promise resolves?

Because at Step 1 in the execution_table, the promise is still pending, so Svelte renders the pending block UI.

What happens if the promise rejects after showing the pending block?

At Step 3, the promise state changes to rejected, so Svelte switches to render the catch block UI showing the error.

Can the then block run without the promise resolving?

No, as shown in Step 2, the then block only renders after the promise is fulfilled with data.

Visual Quiz - 3 Questions

Look at the execution_table, what UI is shown at Step 1?

A <p>Loading...</p>

B <p>Data: {data}</p>

C <p>Error: {error.message}</p>

D No UI shown

💡 Hint

Check the 'UI Shown' column for Step 1 in the execution_table

At which step does the promise reject and show an error message?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Promise State' and 'Block Rendered' columns in the execution_table

If the promise never resolves or rejects, which block stays visible?

A Then block

B Catch block

C Pending block

D No block

💡 Hint

Refer to the 'Promise State' and 'Block Rendered' at Step 1 in the execution_table

Concept Snapshot

Await blocks in Svelte watch a promise.
Show pending UI while waiting.
Show then UI on success with data.
Show catch UI on error.
Syntax: {#await promise}...{:then data}...{:catch error}...{/await}

Full Transcript

In Svelte, the await block lets you handle promises easily in your UI. When you start a promise, the block shows a pending message. Once the promise finishes successfully, it shows the data using the then block. If the promise fails, it shows an error message using the catch block. This flow helps keep your UI responsive and clear about loading states. The execution table shows how the UI changes step by step as the promise state changes from pending to fulfilled or rejected.