Stream types (Readable, Writable, Transform, Duplex) in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a readable stream from a string.

Node.js

const { Readable } = require('stream');
const readable = [1] Readable({
  read() {
    this.push('Hello');
    this.push(null);
  }
});

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

Acreate

Bnew

Cfrom

Dpipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'from' expects an iterable argument, not an options object with read().

Trying to use 'pipe' here is incorrect.

The new Readable() constructor creates a readable stream instance where you can define a custom read() method to push data like strings.

2fill in blank

medium

Complete the code to write data to a writable stream.

Node.js

const { Writable } = require('stream');
const writable = new Writable({
  write(chunk, encoding, callback) {
    console.log(chunk.toString());
    [1]();
  }
});

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

Aend

Bwrite

Ccallback

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Calling 'write' inside 'write' causes infinite recursion.

Forgetting to call 'callback' causes the stream to hang.

The write method must call the callback function to signal that it finished processing the chunk.

3fill in blank

hard

Fix the error in the transform stream by completing the missing method call.

Node.js

const { Transform } = require('stream');
const transform = new Transform({
  transform(chunk, encoding, callback) {
    this.push(chunk.toString().toUpperCase());
    [1]();
  }
});

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

Apush

Bend

Cwrite

Dcallback

Attempts:

3 left

💡 Hint

Common Mistakes

Calling 'write' or 'push' instead of 'callback' causes errors.

Forgetting to call 'callback' causes the stream to stall.

In a transform stream, the transform method must call callback() to signal that the chunk has been processed.

4fill in blank

hard

Fill both blanks to create a duplex stream that echoes input data.

Node.js

const { Duplex } = require('stream');
const duplex = new Duplex({
  read(size) {
    // No data to push here
  },
  write(chunk, encoding, [1]) {
    this.push(chunk);
    [2]();
  }
});

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

Acallback

Bwrite

Cend

Dread

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'write' or 'end' instead of 'callback' causes errors.

Not calling the callback causes the stream to hang.

In the write method of a duplex stream, the third argument is the callback function that must be called to signal completion.

5fill in blank

hard

Fill all three blanks to create a transform stream that reverses input strings.

Node.js

const { Transform } = require('stream');
const reverse = new Transform({
  transform(chunk, encoding, [1]) {
    const input = chunk.toString();
    const reversed = input.split('').[2]().join('');
    this.push(reversed);
    [3]();
  }
});

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

Acallback

Breverse

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reverse' as a function argument name causes errors.

Forgetting to call callback causes the stream to stall.

The transform method uses the callback function to signal completion. The string is reversed by splitting, reversing, and joining. Both {{BLANK_1}} and {{BLANK_3}} are the callback function.

Practice

1. Which type of Node.js stream is designed to only provide data piece by piece for reading?

easy

A. Transform stream

B. Readable stream

C. Writable stream

D. Duplex stream

Solution

1. Step 1: Understand stream roles

   Readable streams are designed to emit data chunks for consumption.

2. Step 2: Match stream type to description

   Only readable streams provide data piece by piece without accepting input.

3. Final Answer:

   Readable stream -> Option B

4. Quick Check:

   Readable = provides data [OK]

Hint: Readable streams only output data, no input accepted [OK]

Common Mistakes:

• Confusing writable streams as data providers
• Thinking transform streams only read data
• Mixing duplex with readable-only behavior

2. Which of the following is the correct way to create a writable stream in Node.js?

easy

A. const stream = new Readable();

B. const stream = new Duplex();

C. const stream = new Transform();

D. const stream = new Writable({ write(chunk, encoding, callback) { callback(); } });

Solution

1. Step 1: Identify writable stream creation

   Writable streams require a write method to handle incoming data chunks.

2. Step 2: Check options for correct syntax

   const stream = new Writable({ write(chunk, encoding, callback) { callback(); } }); correctly creates a Writable stream with a write method and callback.

3. Final Answer:

   const stream = new Writable({ write(chunk, encoding, callback) { callback(); } }); -> Option D

4. Quick Check:

   Writable needs write() method [OK]

Hint: Writable streams need a write() method in options [OK]

Common Mistakes:

• Using Readable constructor for writable stream
• Omitting write method in Writable options
• Confusing Transform or Duplex constructors

3. What will be the output of the following code snippet?

const { Transform } = require('stream');
const upperCase = new Transform({
  transform(chunk, encoding, callback) {
    this.push(chunk.toString().toUpperCase());
    callback();
  }
});

upperCase.on('data', data => console.log(data.toString()));
upperCase.write('hello');
upperCase.end();

medium

A. HELLO

B. hello

C. undefined

D. Error: callback not called

Solution

1. Step 1: Understand Transform stream behavior

   The transform method converts input chunk to uppercase and pushes it forward.

2. Step 2: Analyze event and output

   The 'data' event logs the transformed chunk, which is uppercase 'HELLO'.

3. Final Answer:

   HELLO -> Option A

4. Quick Check:

   Transform changes data to uppercase [OK]

Hint: Transform streams modify data before output [OK]

Common Mistakes:

• Expecting original lowercase output
• Forgetting to call callback() in transform
• Assuming no output without explicit read

4. Identify the error in this Duplex stream implementation:

const { Duplex } = require('stream');
const duplex = new Duplex({
  read(size) {
    this.push('data');
  },
  write(chunk, encoding, callback) {
    console.log(chunk.toString());
    callback();
  }
});
duplex.on('data', data => console.log('Received:', data.toString()));
duplex.write('hello');

medium

A. The Duplex constructor is missing the object mode option

B. The write method is missing the callback call

C. The read method should push null to signal end of data

D. The duplex stream cannot both read and write

Solution

1. Step 1: Check read method behavior

   The read method pushes 'data' but never signals end by pushing null.

2. Step 2: Understand stream end signaling

   Streams must push null to indicate no more data; missing here causes hanging.

3. Final Answer:

   The read method should push null to signal end of data -> Option C

4. Quick Check:

   read() must push null to end stream [OK]

Hint: read() must push null to end data stream [OK]

Common Mistakes:

• Not pushing null in read method
• Forgetting callback in write method
• Assuming Duplex can't read and write

5. You want to create a custom Duplex stream that reads numbers from 1 to 3 and writes their squares. Which approach correctly implements this behavior?

hard

A. Push numbers 1 to 3 in read(), write() logs squares, and push null after 3

B. Push squares in read(), write() pushes numbers 1 to 3, no null needed

C. Use Transform stream instead of Duplex for this task

D. Only Writable stream can handle both reading and writing

Solution

1. Step 1: Understand Duplex stream roles

   Duplex streams can read and write independently; read() pushes data, write() processes input.

2. Step 2: Match behavior to implementation

   Push numbers 1 to 3 in read(), write() logs squares, and push null after last number to end stream.

3. Final Answer:

   Push numbers 1 to 3 in read(), write() logs squares, and push null after 3 -> Option A

4. Quick Check:

   Duplex reads numbers, writes squares [OK]

Hint: Duplex streams read and write independently [OK]

Common Mistakes:

• Confusing Transform with Duplex for this task
• Not pushing null to end read stream
• Thinking Writable streams can read data