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A **Transform Stream** is a special kind of Node.js stream that is both **Readable** and **Writable**,
but unlike a plain `Duplex`, it **transforms** the data passing through it.

> In other words: **a Transform Stream = a Duplex Stream + data transformation.**

## The concept

A `Transform Stream` takes input data → processes it → outputs a modified result.
It's used when you need to **process a data stream on the fly**, without waiting for it to end.

## Examples of built-in Transform Streams

| Module | Example | Description |
|---|---|---|
| `zlib` | `zlib.createGzip()` / `zlib.createGunzip()` | Compressing and decompressing data |
| `crypto` | `crypto.createCipheriv()` / `createDecipheriv()` | Encryption/decryption |
| `stream` | `new stream.Transform()` | Building your own transform stream |
| `readline` | Line-by-line text transformations | |

## A simple analogy

Picture a conveyor belt:

- a stream of lines goes in,
- the stream transforms them (for example, calls `toUpperCase()`),
- and outputs the modified data.

## A custom Transform Stream example

```javascript
const { Transform } = require('stream');

class UpperCaseTransform extends Transform {
  constructor(options) {
    super(options);
  }

_transform(chunk, encoding, callback) {
    const transformed = chunk.toString().toUpperCase();
    this.push(transformed); // pass the result along
    callback(); // signal that processing is done
  }
}

const upperCase = new UpperCaseTransform();

process.stdin.pipe(upperCase).pipe(process.stdout);
```

Here:

- `process.stdin` is the input stream (Readable),
- `upperCase` is our Transform stream (modifies the data),
- `process.stdout` is the output stream (Writable).

Anything you type into the console prints back in uppercase.

## The main Transform Stream methods

| Method | Description |
|---|---|
| `_transform(chunk, encoding, callback)` | The main method, transforms the input data |
| `_flush(callback)` | Called once the input stream ends, lets you "push out" any leftover data |
| `.push(data)` | Passes processed data to the output stream |

## An example with `_flush()`

```javascript
class JSONLinesTransform extends Transform {
  constructor() {
    super({ readableObjectMode: true });
    this.buffer = '';
  }

_transform(chunk, encoding, callback) {
    this.buffer += chunk.toString();
    let lines = this.buffer.split('\n');
    this.buffer = lines.pop(); // keep the last, incomplete line
    for (const line of lines) {
      if (line.trim()) {
        this.push(JSON.parse(line));
      }
    }
    callback();
  }

_flush(callback) {
    if (this.buffer.trim()) {
      this.push(JSON.parse(this.buffer));
    }
    callback();
  }
}
```

This stream:

- reads **JSON lines** one by one,
- turns them into **JavaScript objects**,
- and passes them along as a stream of objects.

## Typical Transform Stream use cases

| Task | Example |
|---|---|
| Compressing files | `fs.createReadStream('file.txt').pipe(zlib.createGzip()).pipe(fs.createWriteStream('file.gz'))` |
| Encryption | `fs.createReadStream('data').pipe(crypto.createCipheriv(...)).pipe(fs.createWriteStream('data.enc'))` |
| Formatting data | JSON → CSV, XML → JSON, and so on |
| Logging / filtering / debouncing | Streaming transformation of logs, HTTP requests, sensor data |

## The difference between Duplex and Transform

| Property | Duplex | Transform |
|---|---|---|
| Reading and writing | Yes | Yes |
| Transforms data | No | Yes |
| Input and output are linked | No | Yes |
| Example | A TCP socket | gzip, a cipher, a JSON parser |

## Advantages of Transform Streams

- Asynchronous processing on the fly
- Memory savings, no need to hold the whole file in RAM
- Simple composition (`pipe()` chains)
- Easy to extend, custom transformers are simple to write

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