Streaming with promises in Svelte

Introduction

Streaming with promises lets you show data bit by bit as it arrives, instead of waiting for everything to load. This makes apps feel faster and smoother.

Loading large files or images progressively so users see content sooner.

Fetching live data feeds like chat messages or notifications.

Displaying search results as they come in without waiting for all results.

Showing video or audio streams that start playing immediately.

Handling slow network responses by updating UI step-by-step.

Syntax

Svelte

let dataPromise = fetch('url').then(response => {
  const reader = response.body.getReader();
  return new ReadableStream({
    start(controller) {
      function push() {
        reader.read().then(({ done, value }) => {
          if (done) {
            controller.close();
            return;
          }
          controller.enqueue(value);
          push();
        });
      }
      push();
    }
  });
});

This code uses fetch to get data and reads it in chunks using a ReadableStream.

The reader.read() method returns a promise that resolves with a chunk of data or signals the end.

Examples

Simple example getting the raw stream from a fetch response.

Svelte

let streamPromise = fetch('/api/data').then(res => res.body);

This example reads the stream chunk by chunk and converts it to text.

Svelte

let textStream = fetch('/api/text').then(async res => {
  const reader = res.body.getReader();
  let result = '';
  while(true) {
    const { done, value } = await reader.read();
    if (done) break;
    result += new TextDecoder().decode(value, { stream: true });
  }
  return result;
});

This example collects all chunks and parses them as JSON after streaming.

Svelte

let jsonStream = fetch('/api/json').then(async res => {
  const reader = res.body.getReader();
  let chunks = [];
  while(true) {
    const { done, value } = await reader.read();
    if (done) break;
    chunks.push(value);
  }
  const full = new Uint8Array(chunks.flatMap(c => Array.from(c)));
  return JSON.parse(new TextDecoder().decode(full));
});

Sample Program

This Svelte component fetches a post and streams its text content. It updates the UI as chunks arrive, showing a loading message until done.

Svelte

<script>
  import { onMount } from 'svelte';
  let streamedText = '';
  let loading = true;
  onMount(async () => {
    const response = await fetch('https://jsonplaceholder.typicode.com/posts/1');
    const reader = response.body.getReader();
    const decoder = new TextDecoder();
    while(true) {
      const { done, value } = await reader.read();
      if (done) break;
      streamedText += decoder.decode(value, { stream: true });
    }
    loading = false;
  });
</script>

<main>
  {#if loading}
    <p>Loading content...</p>
  {:else}
    <pre>{streamedText}</pre>
  {/if}
</main>

Important Notes

Streaming lets users see data faster, but handling partial data requires careful UI updates.

Always handle errors and network issues when working with streams.

Use TextDecoder with the stream: true option to decode chunks properly.

Summary

Streaming with promises helps show data bit by bit for faster user feedback.

Use fetch with ReadableStream and reader.read() to get chunks.

Update your UI as chunks arrive to improve user experience.

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