Streaming with promises in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a promise in JavaScript?

A promise is an object that represents the eventual completion or failure of an asynchronous operation and its resulting value.

intermediate

How does streaming with promises improve user experience in Svelte?

Streaming with promises allows parts of the UI to render as soon as data is available, making the app feel faster and more responsive.

beginner

In Svelte, how can you use a promise to show loading and then display data?

Use the {#await promise} block to show a loading state, then display the resolved data inside the then section of the {#await} block.

intermediate

What happens if a promise rejects inside a Svelte {#await} block?

You can handle errors by adding a catch block inside {#await} to show an error message or fallback UI.

beginner

Why is streaming with promises better than waiting for all data before rendering?

Streaming lets the app show parts of the UI immediately as data arrives, reducing wait time and improving perceived speed.

In Svelte, which block is used to handle promises for streaming data?

A {#await}

B {#if}

C {#each}

D {#then}

The {#await} block is designed to handle promises, showing loading, then resolved data or errors.

What does the 'then' section inside a Svelte {#await} block do?

A Shows loading state

B Handles errors

C Cancels the promise

D Displays resolved promise data

The 'then' section runs when the promise resolves successfully and displays the data.

How can you show an error message if a promise fails in Svelte?

A Use a catch block inside {#await}

B Use an else block

C Use a try-catch statement in markup

D Use a separate component

The catch block inside {#await} handles rejected promises and shows error UI.

Why is streaming with promises useful in web apps?

A It blocks user interaction

B It delays rendering until all data loads

C It allows showing UI parts as data arrives

D It caches all data locally

Streaming lets the app render parts immediately, improving user experience.

Which of these is NOT a benefit of streaming with promises?

A Faster perceived loading

B Guarantees data arrives instantly

C Immediate UI updates

D Better error handling

Streaming improves perceived speed but cannot make data arrive instantly.

Explain how you would use Svelte's {#await} block to stream data from a promise, including loading and error states.

Think about the three parts: loading, success, and error.

Describe why streaming with promises can make a web app feel faster and more responsive.

Compare waiting for all data vs showing partial data early.

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