read a directory and write the output into a file using streams

• Avoid doing inline require - always put require at the top
• Readable.from is a convenient way to turn a data structure (string, array) into a readable stream
• It's not needed in your directory processing example, and really neither is the transform stream
• A better example to use with your transform stream would be a TCP socket:

const { Transform, pipeline } = require('stream')
 
const streamUpper = () => {
  return new Transform({
    transform(chunk, enc, next) {
        next(null, chunk.toString().toUpperCase())
    }   
  })
}
 
const { createServer } = require('net')
 
createServer((socket) => {
  pipeline(socket, streamUpper(), socket, (err) => {
    if (err) console.error(err)
    else console.log('socket closed')
  })
}).listen(9999)

Readable.from would be useful for testing your stream:

const { Transform, Readable } = require('stream')
const assert = require('assert').strict
const streamUpper = () => {
  return new Transform({
    transform(chunk, enc, next) {
        next(null, chunk.toString().toUpperCase())
    }   
  })
}
 
const upper = streamUpper()
Readable.from(['test-a', 'test-b']).pipe(upper)
upper.once('data', (first) => {
  assert.equal(first, 'TEST-A')
  upper.once('data', (second) {
    assert.equal(first, 'TEST-B')
  })
})