Streaming with promises in Svelte - Mini Project: Build & Apply

Streaming with promises in Svelte

📖 Scenario: You are building a simple Svelte app that fetches a list of messages from a server. The server sends the messages one by one as a stream. You want to show each message as soon as it arrives, without waiting for the entire list.

🎯 Goal: Create a Svelte component that uses a promise to stream messages and display them live as they arrive.

📋 What You'll Learn

Create an array to hold messages

Create a promise that simulates streaming messages

Use await block to handle the promise and update the UI as messages arrive

Display each message inside a list in the component

💡 Why This Matters

🌍 Real World

Streaming data is common in chat apps, live notifications, or real-time dashboards where you want to show data as it arrives without waiting for everything.

💼 Career

Understanding how to handle promises and streams in frameworks like Svelte is important for building responsive, user-friendly web apps that handle asynchronous data efficiently.

1

Set up the messages array

Create a variable called messages and set it to an empty array [] inside the <script> tag.

Svelte

// Create a variable called messages and set it to []
// Your code here

Hint

Use let messages = []; to create an empty array for messages.

2

Create a streaming promise

Create a variable called streamPromise and assign it a new Promise that simulates streaming messages. Inside the promise, use setTimeout to add these messages to messages one by one every 500ms: 'Hello', 'from', 'Svelte'. Resolve the promise after all messages are added.

Svelte

let messages = [];

// Create streamPromise that adds messages with delays
// Your code here

Hint

Use a recursive function with setTimeout inside the promise to add messages one by one.

3

Use await block to show messages

Inside the <ul> tag, use a Svelte {#await} block with streamPromise. While waiting, show <li>Loading...</li>. When resolved, display each message in messages as <li> items using {#each} with message as the iterator variable.

Svelte

let messages = [];

let streamPromise = new Promise((resolve) => {
  const data = ['Hello', 'from', 'Svelte'];
  let index = 0;
  function sendNext() {
    if (index 


  {#await streamPromise}
    Loading...
  {:then}
    
    
  {/await}

Hint

Use {#await streamPromise} with {#each messages as message} inside the :then block to show messages.

4

Add accessibility and finalize

Add an aria-live="polite" attribute to the <ul> tag to announce new messages to screen readers. Also, add a role="list" attribute to the <ul> for better accessibility.

Svelte

let messages = [];

let streamPromise = new Promise((resolve) => {
  const data = ['Hello', 'from', 'Svelte'];
  let index = 0;
  function sendNext() {
    if (index 



  {#await streamPromise}
    Loading...
  {:then}
    {#each messages as message}
      {message}
    {/each}
  {/await}

Hint

Add aria-live="polite" and role="list" attributes to the <ul> tag for accessibility.

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