Streaming long operations in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a streaming response in a Next.js server component.

NextJS

export async function GET() {
  const stream = new ReadableStream({
    start(controller) {
      controller.enqueue(new TextEncoder().encode('Hello, '));
      controller.enqueue(new TextEncoder().encode('world!'));
      controller.close();
    }
  });
  return new Response(stream, { headers: { 'Content-Type': [1] } });
}

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

A'text/plain'

B'text/html'

C'application/json'

D'application/octet-stream'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'application/json' when the content is plain text.

Forgetting to set the Content-Type header.

The streaming response sends plain text, so the Content-Type should be 'text/plain'.

2fill in blank

medium

Complete the code to create a readable stream that sends chunks with a delay.

NextJS

export async function GET() {
  const stream = new ReadableStream({
    async start(controller) {
      controller.enqueue(new TextEncoder().encode('Loading'));
      await new Promise(resolve => setTimeout(resolve, [1]));
      controller.enqueue(new TextEncoder().encode('...done'));
      controller.close();
    }
  });
  return new Response(stream, { headers: { 'Content-Type': 'text/plain' } });
}

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

Anull

B'1000'

C1000

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string instead of a number to setTimeout.

Using null or undefined which causes errors.

The setTimeout delay expects a number in milliseconds, so 1000 means 1 second delay.

3fill in blank

hard

Fix the error in the streaming function by completing the blank.

NextJS

export async function GET() {
  const encoder = new TextEncoder();
  const stream = new ReadableStream({
    start(controller) {
      controller.enqueue(encoder.encode('Start'));
      controller.enqueue(encoder.encode('Middle'));
      controller.[1]();
    }
  });
  return new Response(stream, { headers: { 'Content-Type': 'text/plain' } });
}

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

Aend

Bclose

Cfinish

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'end' or 'finish' which are not valid methods.

Forgetting to close the stream causing it to hang.

The ReadableStream controller uses the 'close' method to signal the end of the stream.

4fill in blank

hard

Fill both blanks to create a streaming response that sends JSON chunks.

NextJS

export async function GET() {
  const stream = new ReadableStream({
    start(controller) {
      controller.enqueue(new TextEncoder().encode(JSON.[1]({ message: 'Hello' })));
      controller.enqueue(new TextEncoder().encode(JSON.[2]({ message: 'World' })));
      controller.close();
    }
  });
  return new Response(stream, { headers: { 'Content-Type': 'application/json' } });
}

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

Astringify

Bparse

CtoString

Dformat

Attempts:

3 left

💡 Hint

Common Mistakes

Using JSON.parse which causes errors.

Using toString which does not produce JSON.

To send JSON as text, use JSON.stringify to convert objects to strings.

5fill in blank

hard

Fill all three blanks to create a streaming response that sends multiple delayed chunks.

NextJS

export async function GET() {
  const stream = new ReadableStream({
    async start(controller) {
      controller.enqueue(new TextEncoder().encode('Step 1'));
      await new Promise(resolve => setTimeout(resolve, [1]));
      controller.enqueue(new TextEncoder().encode('Step 2'));
      await new Promise(resolve => setTimeout(resolve, [2]));
      controller.enqueue(new TextEncoder().encode('Step 3'));
      controller.[3]();
    }
  });
  return new Response(stream, { headers: { 'Content-Type': 'text/plain' } });
}

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

A500

B1000

Cclose

Dfinish

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'finish' instead of 'close' to end the stream.

Using strings instead of numbers for delays.

The first delay is 1000ms (option B), the second delay is 1000ms (option B) but since only one B is allowed, the first delay is 500ms (A), second 1000ms (B), and the stream is closed with 'close' (C).

Practice

1. What is the main benefit of using streaming in Next.js for long operations?

easy

A. Users see data progressively, reducing perceived wait time.

B. It automatically caches all data on the server.

C. It compresses data to reduce bandwidth usage.

D. It prevents any errors during data fetching.

Solution

1. Step 1: Understand streaming purpose

   Streaming sends data in chunks so users can see progress during long operations.

2. Step 2: Identify user experience impact

   This reduces the perceived wait time by showing partial results early.

3. Final Answer:

   Users see data progressively, reducing perceived wait time. -> Option A

4. Quick Check:

   Streaming improves user experience = A [OK]

Hint: Streaming shows partial data early to users [OK]

Common Mistakes:

• Thinking streaming caches data automatically
• Confusing streaming with data compression
• Assuming streaming prevents all errors

2. Which syntax correctly creates a streaming response in a Next.js API route?

easy

A. const stream = new ReadableStream({ start(controller) { controller.enqueue('data'); controller.close(); } }); return new Response(stream);

B. const stream = new StreamReader(); stream.push('data'); return stream;

C. const stream = new WritableStream(); stream.write('data'); return stream;

D. const stream = new ReadableStream(); stream.send('data'); return stream;

Solution

1. Step 1: Recall correct ReadableStream usage

   ReadableStream requires a start method with a controller to enqueue data and close the stream.

2. Step 2: Match syntax with Next.js streaming pattern

   const stream = new ReadableStream({ start(controller) { controller.enqueue('data'); controller.close(); } }); return new Response(stream); uses controller.enqueue and controller.close correctly and returns a Response wrapping the stream.

3. Final Answer:

   const stream = new ReadableStream({ start(controller) { controller.enqueue('data'); controller.close(); } }); return new Response(stream); -> Option A

4. Quick Check:

   ReadableStream with controller.enqueue = B [OK]

Hint: Use ReadableStream with start(controller) and enqueue [OK]

Common Mistakes:

• Using WritableStream instead of ReadableStream
• Calling non-existent methods like send() or push()
• Not closing the stream after enqueue

3. Given this Next.js API route code, what will the client receive?

export async function GET() {
  const stream = new ReadableStream({
    async start(controller) {
      controller.enqueue(new TextEncoder().encode('Hello '));
      await new Promise(r => setTimeout(r, 1000));
      controller.enqueue(new TextEncoder().encode('World!'));
      controller.close();
    }
  });
  return new Response(stream);
}

medium

A. Only "Hello " is received; stream never closes.

B. "Hello World!" received in two chunks with 1 second delay between.

C. "Hello World!" received all at once after 1 second delay.

D. Syntax error due to missing await in controller.enqueue.

Solution

1. Step 1: Analyze streaming behavior

   The stream first sends "Hello " immediately, then waits 1 second before sending "World!".

2. Step 2: Understand client reception

   The client receives data in two chunks with a delay, then the stream closes properly.

3. Final Answer:

   "Hello World!" received in two chunks with 1 second delay between. -> Option B

4. Quick Check:

   Streaming sends chunks with delay = C [OK]

Hint: Streaming sends chunks as they enqueue, not all at once [OK]

Common Mistakes:

• Assuming all data arrives after delay
• Thinking stream never closes
• Confusing async enqueue with syntax error

4. What is wrong with this Next.js streaming code snippet?

export async function GET() {
  const stream = new ReadableStream({
    start(controller) {
      controller.enqueue('Data chunk');
      // Missing controller.close()
    }
  });
  return new Response(stream);
}

medium

A. ReadableStream cannot be used in Next.js API routes.

B. controller.enqueue must be async function.

C. The stream never closes, causing the client to wait indefinitely.

D. Response must be wrapped in JSON.stringify.

Solution

1. Step 1: Check stream lifecycle

   The code enqueues data but does not call controller.close(), so the stream stays open.

2. Step 2: Understand client impact

   The client waits forever for the stream to end, causing a hang or timeout.

3. Final Answer:

   The stream never closes, causing the client to wait indefinitely. -> Option C

4. Quick Check:

   Missing controller.close() = stream hangs [OK]

Hint: Always call controller.close() after enqueueing all data [OK]

Common Mistakes:

• Forgetting to close the stream
• Thinking enqueue must be async
• Misunderstanding Response usage

5. You want to stream a large list of items from a Next.js API route, sending each item as JSON chunk. Which approach correctly streams the data without blocking the client?

hard

A. Send each item with separate fetch requests instead of streaming.

B. Convert entire list to JSON string, enqueue once, then close the stream.

C. Use WritableStream to write each JSON chunk and return it directly.

D. Use ReadableStream with async start(controller), loop over items, enqueue JSON strings, and close after all.

Solution

1. Step 1: Understand streaming large data

   Streaming each item as JSON chunk progressively sends data without waiting for all items.

2. Step 2: Identify correct streaming pattern

   Using ReadableStream with async start allows looping over items, enqueueing each chunk, then closing the stream.

3. Step 3: Evaluate other options

   A requires multiple requests (inefficient); B sends all data at once (no streaming benefit); C uses wrong WritableStream type.

4. Final Answer:

   Use ReadableStream with async start(controller), loop over items, enqueue JSON strings, and close after all. -> Option D

5. Quick Check:

   Stream chunks in loop with ReadableStream = D [OK]

Hint: Stream each item chunk inside async start, then close stream [OK]

Common Mistakes:

• Sending all data at once, losing streaming benefit
• Using WritableStream instead of ReadableStream
• Making multiple fetch calls instead of streaming