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.