Streaming with promises in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a promise that resolves after 2 seconds.

Svelte

const dataPromise = new Promise((resolve) => {
  setTimeout(() => {
    resolve([1]);
  }, 2000);
});

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

A'Data loaded'

Bconsole.log('Done')

Creject('Error')

DsetTimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Using reject instead of resolve

Calling console.log inside resolve

Passing setTimeout instead of a value

The promise resolves with the string 'Data loaded' after 2 seconds.

2fill in blank

medium

Complete the code to await the promise and store the result in a variable.

Svelte

async function loadData() {
  const result = await [1];
  return result;
}

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

AfetchData()

BsetTimeout()

CdataPromise

DPromise.resolve()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling fetchData() which is undefined

Using setTimeout() which is not a promise

Using Promise.resolve() without data

We await the previously created dataPromise to get the resolved data.

3fill in blank

hard

Fix the error in the Svelte component to show loading text while waiting for the promise.

Svelte

<script>
  let data = null;
  let loading = true;

  dataPromise.then(value => {
    data = value;
    [1] = false;
  });
</script>

{#if loading}
  <p>Loading...</p>
{:else}
  <p>{data}</p>
{/if}

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

AdataPromise

Bdata

Cvalue

Dloading

Attempts:

3 left

💡 Hint

Common Mistakes

Setting data to false instead of loading

Using value or dataPromise incorrectly

Not updating loading state

We set loading to false after the promise resolves to update the UI.

4fill in blank

hard

Fill both blanks to create a reactive statement that updates when the promise resolves.

Svelte

<script>
  let data = null;
  let loading = true;

  $: [1] = loading ? 'Loading...' : data;

  dataPromise.then(value => {
    data = value;
    [2] = false;
  });
</script>

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

AdisplayText

Bloading

Cdata

DisLoading

Attempts:

3 left

💡 Hint

Common Mistakes

Using data instead of displayText in reactive statement

Setting isLoading instead of loading

Not using reactive syntax

The reactive variable displayText shows loading or data based on loading.

5fill in blank

hard

Fill all three blanks to create a Svelte component that streams data chunks and updates the UI.

Svelte

<script>
  import { onMount } from 'svelte';
  let chunks = [];
  let loading = true;

  async function streamData() {
    const response = await fetch('/stream');
    const reader = response.body.getReader();
    while(true) {
      const { done, value } = await reader.read();
      if(done) break;
      chunks = [...chunks, new TextDecoder().[1](value)];
    }
    [2] = false;
  }

  onMount(() => {
    [3]();
  });
</script>

{#if loading}
  <p>Loading stream...</p>
{:else}
  <p>{chunks.join('')}</p>
{/if}

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

Adecode

Bloading

CstreamData

Dread

Attempts:

3 left

💡 Hint

Common Mistakes

Using read instead of decode

Not setting loading to false

Not calling streamData on mount

The decode method converts bytes to text, loading is set false after streaming, and streamData is called on mount.

Practice

1. What is the main benefit of using streaming with promises in Svelte when fetching data?

easy

A. It allows showing data bit by bit for faster user feedback.

B. It delays showing any data until the entire response is received.

C. It automatically caches all data without extra code.

D. It prevents any UI updates until the promise fully resolves.

Solution

1. Step 1: Understand streaming with promises

   Streaming lets you process data as it arrives, not waiting for all data.

2. Step 2: Connect streaming to user experience

   Showing data bit by bit gives faster feedback, improving user experience.

3. Final Answer:

   It allows showing data bit by bit for faster user feedback. -> Option A

4. Quick Check:

   Streaming = faster feedback [OK]

Hint: Streaming means showing data as it arrives, not waiting [OK]

Common Mistakes:

• Thinking streaming waits for full data
• Confusing streaming with caching
• Believing UI updates only after full promise resolves

2. Which of the following is the correct way to start reading chunks from a fetch response stream in Svelte?

easy

A. const reader = response.getReader();

B. const reader = response.body.getReader();

C. const reader = response.readableStream();

D. const reader = response.streamReader();

Solution

1. Step 1: Identify how to access the stream reader

   The fetch response has a body property which is a ReadableStream.

2. Step 2: Use the correct method to get the reader

   Calling getReader() on response.body returns the reader.

3. Final Answer:

   const reader = response.body.getReader(); -> Option B

4. Quick Check:

   Use response.body.getReader() [OK]

Hint: Remember: reader comes from response.body.getReader() [OK]

Common Mistakes:

• Calling getReader() directly on response
• Using non-existent methods like readableStream()
• Confusing response.body with response itself

3. Given this Svelte code snippet, what will be logged to the console?

async function fetchData() {
  const response = await fetch('/data');
  const reader = response.body.getReader();
  const { value, done } = await reader.read();
  console.log(new TextDecoder().decode(value));
}
fetchData();

medium

A. The first chunk of data from the response decoded as text.

B. The entire response body as a string.

C. An error because reader.read() returns a promise without value.

D. Undefined because value is not set.

Solution

1. Step 1: Understand reader.read() output

   reader.read() returns a promise resolving to an object with value (chunk) and done (boolean).

2. Step 2: Decode the first chunk

   The code decodes the first chunk's bytes to text and logs it.

3. Final Answer:

   The first chunk of data from the response decoded as text. -> Option A

4. Quick Check:

   reader.read() gives first chunk [OK]

Hint: reader.read() returns first chunk and done flag [OK]

Common Mistakes:

• Assuming reader.read() returns full response
• Expecting reader.read() to return undefined value
• Confusing promise with direct value

4. What is wrong with this Svelte code that tries to stream data from fetch?

async function load() {
  const response = await fetch('/api');
  const reader = response.getReader();
  while(true) {
    const { value, done } = await reader.read();
    if(done) break;
    console.log(value);
  }
}

medium

A. Using while(true) causes infinite loop.

B. Missing await before fetch call.

C. Calling getReader() directly on response instead of response.body.

D. Not decoding value before logging.

Solution

1. Step 1: Check how getReader() is called

   getReader() must be called on response.body, not on response itself.

2. Step 2: Identify the error cause

   Calling response.getReader() will cause a runtime error because response has no such method.

3. Final Answer:

   Calling getReader() directly on response instead of response.body. -> Option C

4. Quick Check:

   Use response.body.getReader() [OK]

Hint: getReader() is on response.body, not response [OK]

Common Mistakes:

• Calling getReader() on response
• Forgetting to decode chunks
• Thinking while(true) always causes infinite loop

5. You want to display a loading message that updates with each chunk of streamed data in Svelte. Which approach correctly updates the UI as chunks arrive?

hard

A. Update a local variable inside the async function without reactive binding.

B. Wait for all chunks, then update the UI once with the full data.

C. Use setTimeout to delay UI updates until streaming finishes.

D. Use a writable store to append each decoded chunk and bind it to the UI.

Solution

1. Step 1: Understand UI reactivity in Svelte

   Svelte updates the UI when reactive variables or stores change.

2. Step 2: Choose a method to update UI per chunk

   Using a writable store to append each chunk triggers UI updates as data arrives.

3. Step 3: Why other options fail

   Waiting for all chunks delays UI update; setTimeout delays unnecessarily; local variable changes without binding won't update UI.

4. Final Answer:

   Use a writable store to append each decoded chunk and bind it to the UI. -> Option D

5. Quick Check:

   Writable store + binding = live UI updates [OK]

Hint: Use writable store to update UI reactively per chunk [OK]

Common Mistakes:

• Updating local variables without reactive binding
• Waiting for full data before UI update
• Using timers instead of reactive stores