Streaming long operations in NextJS

Introduction

Streaming long operations lets your app show results bit by bit instead of waiting for everything to finish. This makes users feel the app is faster and more responsive.

When loading large amounts of data from a server and you want to show partial results quickly.

When generating content that takes time, like reports or AI responses, and you want to display it as it arrives.

When you want to improve user experience by showing progress or partial content during slow operations.

When working with APIs that support streaming responses and you want to handle data progressively.

Syntax

NextJS

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

This example shows a Next.js API route using a ReadableStream to send data in chunks.

Use controller.enqueue() to send parts of the response, and controller.close() to finish.

Examples

Sends two parts immediately, then closes the stream.

NextJS

export async function GET() {
  const stream = new ReadableStream({
    start(controller) {
      controller.enqueue(new TextEncoder().encode('Part 1 '));
      controller.enqueue(new TextEncoder().encode('Part 2'));
      controller.close();
    }
  });
  return new Response(stream);
}

Sends 'Loading', waits 1 second, then sends '...done'.

NextJS

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

Sample Program

This Next.js API route streams text messages in three parts with 1-second pauses. The client receives updates progressively, improving user experience during long tasks.

NextJS

import { NextResponse } from 'next/server';

export async function GET() {
  const stream = new ReadableStream({
    async start(controller) {
      const encoder = new TextEncoder();
      controller.enqueue(encoder.encode('Starting operation...\n'));
      await new Promise(r => setTimeout(r, 1000));
      controller.enqueue(encoder.encode('Halfway there...\n'));
      await new Promise(r => setTimeout(r, 1000));
      controller.enqueue(encoder.encode('Operation complete!\n'));
      controller.close();
    }
  });

  return new NextResponse(stream, {
    headers: { 'Content-Type': 'text/plain; charset=utf-8' }
  });
}

Important Notes

Streaming works best with clients that can handle partial responses, like browsers or fetch API.

Always set proper Content-Type headers to help clients interpret streamed data.

Use TextEncoder to convert strings to bytes before enqueueing in the stream.

Summary

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

Use ReadableStream and controller.enqueue() in Next.js API routes to stream responses.

Streaming improves user experience by reducing wait perception.

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