Transform streams for processing in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Node.js transform stream?

Consider this transform stream that converts input text to uppercase. What will be printed to the console when piping 'hello world' through it?

Node.js

import { Transform } from 'stream';

const upperCaseTransform = new Transform({
  transform(chunk, encoding, callback) {
    this.push(chunk.toString().toUpperCase());
    callback();
  }
});

process.stdin.pipe(upperCaseTransform).pipe(process.stdout);

// Input: 'hello world' (typed by user)

A hello world

B HELLO WORLD

C error: chunk.toString is not a function

D undefined

💡 Hint

Think about what the transform function does to each chunk of data.

The transform stream converts each chunk to uppercase before pushing it forward. So the output will be the uppercase version of the input.

📝 Syntax

intermediate

Which option correctly implements a transform stream that reverses input strings?

You want to create a transform stream that reverses each chunk of text. Which code snippet is syntactically correct and works as intended?

A new Transform({ transform(chunk, encoding, callback) { this.push(chunk.toString().split('').reverse().join('')); callback(); } })

B new Transform({ transform(chunk, encoding) { this.push(chunk.toString().reverse()); } })

C new Transform({ transform(chunk, encoding, callback) { callback(null, chunk.toString().split('').reverse().join('')); } })

D new Transform({ transform(chunk, encoding, callback) { this.push(chunk.reverse().toString()); callback(); } })

💡 Hint

Remember the transform method signature and how to push transformed data.

Options A and C correctly implement the reversal. A uses this.push(reversed) then callback(). C uses callback(null, reversed), which is equivalent and internally pushes the data. B misses the callback parameter/call (stream hangs) and reverse() on string (invalid). D reverses Buffer directly (no reverse() method, error).

🔧 Debug

advanced

Why does this transform stream cause a runtime error?

Examine this transform stream code. Why does it throw an error when processing input?

Node.js

import { Transform } from 'stream';

const faultyTransform = new Transform({
  transform(chunk, encoding, callback) {
    const data = chunk.toString().toUpperCase();
    this.push(data);
    callback();
  }
});

A chunk is a Buffer, so chunk.toUpperCase() is not a function, causing a TypeError

B Transform constructor requires an options object with readable and writable set explicitly

C this.push must be called after callback, so order is wrong

D Missing call to callback with error argument causes runtime error

💡 Hint

Check the type of chunk and what methods it supports.

chunk is a Buffer object, which does not have a toUpperCase method. You must convert chunk to string first.

❓ state_output

advanced

What is the final output of this transform stream pipeline?

This pipeline reads input, doubles numbers in text, and outputs the result. What will be printed if input is '1 2 3'?

Node.js

import { Transform } from 'stream';
import { Readable } from 'stream';

const doubleNumbers = new Transform({
  transform(chunk, encoding, callback) {
    const input = chunk.toString();
    const doubled = input.split(' ').map(n => Number(n) * 2).join(' ');
    this.push(doubled);
    callback();
  }
});

const inputStream = Readable.from(['1 2 3']);

inputStream.pipe(doubleNumbers).pipe(process.stdout);

A NaN NaN NaN

B 1 2 3

C error: Number is not a function

D 2 4 6

💡 Hint

Look at how numbers are processed and doubled.

The transform splits the string by spaces, converts each to a number, doubles it, then joins back. So '1 2 3' becomes '2 4 6'.

🧠 Conceptual

expert

Which statement about Node.js transform streams is TRUE?

Select the correct statement about how transform streams work in Node.js.

A Transform streams cannot change the size of data chunks passed through them

B Transform streams automatically buffer all data and do not require calling callback()

C Transform streams can modify data chunks and must always call callback() after pushing data

D Transform streams are only readable and cannot be piped to writable streams

💡 Hint

Think about the transform method contract and stream behavior.

Transform streams modify data chunks and must call callback() to signal completion. They can change chunk size and are both readable and writable.

Practice

1. What is the main purpose of a Transform stream in Node.js?

easy

A. To read data from a file without changing it

B. To write data to a file without reading

C. To modify or transform data chunks as they pass through the stream

D. To buffer all data before processing

Solution

1. Step 1: Understand the role of Transform streams

   Transform streams allow you to change data while it flows, unlike plain readable or writable streams.

2. Step 2: Identify the correct purpose

   Only To modify or transform data chunks as they pass through the stream describes modifying data chunks during streaming, which is the key feature of Transform streams.

3. Final Answer:

   To modify or transform data chunks as they pass through the stream -> Option C

4. Quick Check:

   Transform streams change data on the fly = B [OK]

Hint: Transform streams change data chunks during flow [OK]

Common Mistakes:

• Confusing Transform streams with simple readable or writable streams
• Thinking Transform streams buffer all data before processing
• Assuming Transform streams only read or write without modification

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

easy

A. const { Transform } = require('stream'); const myTransform = new Transform({ transform(chunk, encoding, callback) { callback(null, chunk); } });

B. const { Transform } = require('stream'); const myTransform = Transform();

C. const { Transform } = require('stream'); const myTransform = new Transform();

D. const { Transform } = require('stream'); const myTransform = new Transform({ read() {} });

Solution

1. Step 1: Recall Transform stream creation syntax

   Transform streams require a constructor call with an object containing a transform method to process chunks.

2. Step 2: Check each option

   const { Transform } = require('stream'); const myTransform = new Transform({ transform(chunk, encoding, callback) { callback(null, chunk); } }); correctly uses new Transform with a transform method. const { Transform } = require('stream'); const myTransform = new Transform({ read() {} }); incorrectly uses read instead of transform.

3. Final Answer:

   const { Transform } = require('stream'); const myTransform = new Transform({ transform(chunk, encoding, callback) { callback(null, chunk); } }); -> Option A

4. Quick Check:

   Transform streams need a transform method = D [OK]

Hint: Use new Transform({ transform() }) to create transform streams [OK]

Common Mistakes:

• Forgetting to use the new keyword
• Not providing the transform method
• Using read() instead of transform() in options

3. Given the following Transform stream code, what will be the output when the input chunk is the string "hello"?

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. error

D. undefined

Solution

1. Step 1: Analyze the transform function

   The transform method converts the chunk to a string, then to uppercase, and pushes it to the output.

2. Step 2: Determine the output on 'data' event

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

3. Final Answer:

   HELLO -> Option B

4. Quick Check:

   Transform converts input to uppercase = A [OK]

Hint: Transform pushes uppercase chunk, so output is uppercase [OK]

Common Mistakes:

• Expecting original input without transformation
• Confusing push with callback argument
• Missing toString() conversion causing errors

4. Identify the error in the following Transform stream code snippet:

const { Transform } = require('stream');
const reverse = new Transform({
  transform(chunk, encoding, callback) {
    const reversed = chunk.toString().split('').reverse().join('');
    callback(null, reversed);
  }
});

medium

A. The callback should be called with null and the transformed chunk, so no error

B. The transform method is missing the encoding parameter

C. The chunk should not be converted to string before reversing

D. The transformed chunk should be pushed using this.push(), not passed as callback argument

Solution

1. Step 1: Review Transform stream callback usage

   In Transform streams, the transformed data must be pushed using this.push(), not passed as the second argument to callback.

2. Step 2: Identify the mistake in the code

   The code incorrectly calls callback(null, reversed) instead of pushing reversed data and then calling callback().

3. Final Answer:

   The transformed chunk should be pushed using this.push(), not passed as callback argument -> Option D

4. Quick Check:

   Use this.push() for output, callback(null) to signal done = C [OK]

Hint: Push transformed data, then call callback(null) [OK]

Common Mistakes:

• Passing transformed data as callback second argument
• Not calling callback at all
• Ignoring this.push() method

5. You want to create a Transform stream that filters out all chunks containing the word "skip" (case insensitive) and passes through all other chunks unchanged. Which code snippet correctly implements this behavior?

hard

A. const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (chunk.toString().toLowerCase().includes('skip')) { callback(); } else { this.push(chunk); callback(); } } });

B. const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (chunk.includes('skip')) { this.push(chunk); } callback(); } });

C. const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (!chunk.toString().includes('skip')) { this.push(chunk); } callback(null); } });

D. const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (chunk.toString().toLowerCase().indexOf('skip') === -1) { this.push(chunk); callback(); } } });

Solution

1. Step 1: Understand filtering logic

   Chunks containing "skip" (case insensitive) should be ignored (not pushed), others passed through.

2. Step 2: Check each option for correct logic and callback usage

   const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (chunk.toString().toLowerCase().includes('skip')) { callback(); } else { this.push(chunk); callback(); } } }); correctly converts chunk to lowercase, checks for "skip", skips pushing if found, and calls callback in all cases. const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (chunk.includes('skip')) { this.push(chunk); } callback(); } }); pushes chunks containing "skip" (wrong). const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (!chunk.toString().includes('skip')) { this.push(chunk); } callback(null); } }); misses case insensitivity and calls callback with null (acceptable but less consistent). const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (chunk.toString().toLowerCase().indexOf('skip') === -1) { this.push(chunk); callback(); } } }); misses calling callback when chunk contains "skip" (callback not called).

3. Final Answer:

   const filterSkip = new Transform({ transform(chunk, encoding, callback) { if (chunk.toString().toLowerCase().includes('skip')) { callback(); } else { this.push(chunk); callback(); } } }); -> Option A

4. Quick Check:

   Skip chunks with 'skip' word, push others, always call callback = A [OK]

Hint: Call callback always; push only if chunk lacks 'skip' (case insensitive) [OK]

Common Mistakes:

• Not calling callback in all code paths
• Pushing chunks that should be skipped
• Ignoring case sensitivity in filtering