Streaming with ReadableStream and WritableStream in Deno - Mini Project: Build & Apply

Streaming with ReadableStream and WritableStream in Deno

📖 Scenario: You are building a simple data pipeline in Deno that reads text data from a source, processes it, and writes it to a destination using streams.This simulates real-world tasks like reading files, transforming data, and saving results efficiently without loading everything into memory.

🎯 Goal: Create a small Deno program that uses ReadableStream to provide data, a configuration variable to control chunk size, a TransformStream to convert text to uppercase, and a WritableStream to collect and output the transformed data.

📋 What You'll Learn

Create a ReadableStream that emits three text chunks: 'hello', ' ', and 'world'.

Add a configuration variable chunkSize set to 5.

Use a TransformStream to convert all text chunks to uppercase.

Create a WritableStream that collects all chunks into a single string variable result.

💡 Why This Matters

🌍 Real World

Streaming data efficiently is important in web servers, file processing, and real-time applications to handle large data without blocking or using too much memory.

💼 Career

Understanding streams and how to transform and pipe data is a key skill for backend developers, especially when working with modern JavaScript runtimes like Deno or Node.js.

1

Create a ReadableStream with text chunks

Create a ReadableStream called sourceStream that emits the exact text chunks 'hello', ' ', and 'world' in sequence.

Deno

# Your code here to create sourceStream emitting 'hello', ' ', 'world'

Hint

Use new ReadableStream with a start(controller) method. Use controller.enqueue() to send chunks and controller.close() when done.

2

Add a chunk size configuration variable

Add a constant variable called chunkSize and set it to the number 5.

Deno

const sourceStream = new ReadableStream({
  start(controller) {
    controller.enqueue('hello');
    controller.enqueue(' ');
    controller.enqueue('world');
    controller.close();
  }
});

// Your code here to add chunkSize = 5

Hint

Simply declare const chunkSize = 5; below the stream.

3

Create a TransformStream to uppercase text

Create a TransformStream called upperCaseTransform that converts each incoming text chunk to uppercase and passes it on.

Deno

const sourceStream = new ReadableStream({
  start(controller) {
    controller.enqueue('hello');
    controller.enqueue(' ');
    controller.enqueue('world');
    controller.close();
  }
});

const chunkSize = 5;

// Your code here to create upperCaseTransform

Hint

Use new TransformStream with a transform(chunk, controller) method that calls controller.enqueue(chunk.toUpperCase()).

4

Create a WritableStream to collect output

Create a WritableStream called destinationStream that collects all chunks into a variable result (initially empty string). Append each chunk to result in the write(chunk) method.

Deno

const sourceStream = new ReadableStream({
  start(controller) {
    controller.enqueue('hello');
    controller.enqueue(' ');
    controller.enqueue('world');
    controller.close();
  }
});

const chunkSize = 5;

const upperCaseTransform = new TransformStream({
  transform(chunk, controller) {
    controller.enqueue(chunk.toUpperCase());
  }
});

// Your code here to create destinationStream and result

Hint

Declare let result = ''; and create new WritableStream with a write(chunk) method that appends chunk to result.

Practice

1. What is the main purpose of a ReadableStream in Deno?

easy

A. To accept data chunks that can be written

B. To produce data chunks that can be read

C. To store data permanently

D. To convert data into JSON format

Solution

1. Step 1: Understand the role of ReadableStream

   A ReadableStream is designed to produce data chunks that can be read by consumers.

2. Step 2: Differentiate from WritableStream

   A WritableStream accepts data chunks to be written, which is the opposite role.

3. Final Answer:

   To produce data chunks that can be read -> Option B

4. Quick Check:

   ReadableStream produces data = A [OK]

Hint: ReadableStream produces data; WritableStream consumes data [OK]

Common Mistakes:

• Confusing ReadableStream with WritableStream
• Thinking streams store data permanently
• Assuming streams convert data formats automatically

2. Which of the following is the correct way to create a new WritableStream in Deno?

easy

A. const stream = new WritableStream({ write(chunk) { console.log(chunk); } });

B. const stream = WritableStream.write(chunk => console.log(chunk));

C. const stream = new WritableStream.write(chunk => console.log(chunk));

D. const stream = WritableStream({ write(chunk) { console.log(chunk); } });

Solution

1. Step 1: Recall WritableStream constructor syntax

   WritableStream is created with new WritableStream() and an object with a write method.

2. Step 2: Check each option's syntax

   const stream = new WritableStream({ write(chunk) { console.log(chunk); } }); correctly uses new WritableStream({ write(chunk) { ... } }). Others misuse syntax or omit new.

3. Final Answer:

   const stream = new WritableStream({ write(chunk) { console.log(chunk); } }); -> Option A

4. Quick Check:

   WritableStream needs new + write method object = C [OK]

Hint: Use 'new WritableStream' with an object having a write method [OK]

Common Mistakes:

• Omitting 'new' keyword
• Calling WritableStream as a function
• Passing a function directly instead of an object

3. What will the following code output in Deno?

const readable = new ReadableStream({
  start(controller) {
    controller.enqueue('Hello');
    controller.enqueue('World');
    controller.close();
  }
});

const reader = readable.getReader();

(async () => {
  let result = '';
  while(true) {
    const { done, value } = await reader.read();
    if (done) break;
    result += value + ' ';
  }
  console.log(result.trim());
})();

medium

A. Error: reader.read is not a function

B. HelloWorld

C. Hello World

D. undefined

Solution

1. Step 1: Understand the ReadableStream setup

   The stream enqueues two chunks: 'Hello' and 'World', then closes.

2. Step 2: Analyze the reading loop

   The loop reads each chunk, appends it with a space to result, and stops when done is true.

3. Step 3: Output after trimming

   After reading both chunks, result is 'Hello World ' which trims to 'Hello World'.

4. Final Answer:

   Hello World -> Option C

5. Quick Check:

   Chunks read and concatenated with space = D [OK]

Hint: ReadableStream chunks read sequentially, concatenated with spaces [OK]

Common Mistakes:

• Forgetting to await reader.read()
• Not trimming the final string
• Assuming chunks concatenate without spaces

4. Identify the error in this Deno code using streams:

const writable = new WritableStream({
  write(chunk) {
    console.log(chunk.toUpperCase());
  }
});

const writer = writable.getWriter();

await writer.write(123);
await writer.close();

medium

A. write method must be async

B. Missing await before new WritableStream

C. getWriter() is not a function

D. Cannot call toUpperCase on a number

Solution

1. Step 1: Check the write method usage

   The write method calls chunk.toUpperCase(), which only works on strings.

2. Step 2: Analyze the write call argument

   The code writes the number 123, which has no toUpperCase() method, causing a runtime error.

3. Final Answer:

   Cannot call toUpperCase on a number -> Option D

4. Quick Check:

   toUpperCase on number causes error = A [OK]

Hint: Check data types before calling string methods [OK]

Common Mistakes:

• Assuming write must be async
• Thinking getWriter() is invalid
• Missing await on WritableStream creation

5. You want to transform a ReadableStream of text chunks to uppercase and write them to a WritableStream. Which approach correctly implements this in Deno?

hard

A. Pipe the readable through a TransformStream that uppercases chunks, then pipe to writable

B. Directly write chunks from readable to writable without transformation

C. Use readable.getReader() and writable.getWriter(), but forget to release locks

D. Convert chunks to uppercase after writing to writable

Solution

1. Step 1: Understand stream transformation

   To modify data between readable and writable, use a TransformStream that changes chunks to uppercase.

2. Step 2: Use pipeThrough and pipeTo methods

   Pipe the readable stream through the transform, then pipe the result to the writable stream for correct flow and backpressure handling.

3. Step 3: Evaluate other options

   Directly write chunks from readable to writable without transformation skips transformation, C risks locking issues, and D transforms too late after writing.

4. Final Answer:

   Pipe the readable through a TransformStream that uppercases chunks, then pipe to writable -> Option A

5. Quick Check:

   Use TransformStream + pipeThrough + pipeTo = B [OK]

Hint: Use TransformStream between readable and writable for data changes [OK]

Common Mistakes:

• Skipping TransformStream for data modification
• Not handling stream locks properly
• Transforming data after writing