Node.js streams for XML transformations with xml-stream

Question

I'm using xml-stream to read a large XML file. I'd like to:

1. pipe collected elements to a stream
2. optionally, to transform those elements using one or more pipes
3. pipe the result to an http response

Here is xml-stream snippet collecting required elements:

xml.on('endElement: item', function(item) {
  // pipe item to stream
})

How do I build streams for step 1 and 2?

P.S. xml-stream has only console.log examples

UPDATE 1

Here is what I wrote so far:

stream = require('stream');

let liner = new stream.Transform( { objectMode: true } );

liner._transform = function (data, encoding, done) {
  this.push(data);
  console.log(data);
  console.log('======================='); 
  done();
};

let fileStream = fs.createReadStream(fileNames[0]);

let xmlStream = new XmlStream(fileStream);

let counter = 0;

xmlStream.on('endElement: Item', function(el) {
  liner.write(el);
  counter += 1;
});

xmlStream.on('end', function() {
  console.log(counter);
  liner.end();
});

_transform get called on every write, however piping liner stream to http result doesn't produce any output.

Answer 1

Mission accomplished. The function below returns transform stream that can be piped to any writable stream. liner._flush is necessary only you want to add some data in the end of the stream.

P.S. A handy module (not used here) https://github.com/rvagg/through2

const fs = require('fs');
const stream = require('stream');
const XmlStream = require('xml-stream');

function getTransformStream() { 

  let liner = new stream.Transform( { objectMode: true } );

  liner._transform = function (data, encoding, done) {
    // have your transforms here
    this.push(data);
    console.log(data);
    console.log('=======================');
    done();
  };

  liner._flush = function (done) {
    console.log('DONE DONE DONE DONE');
    done();
  };


  let fileStream = fs.createReadStream('filename');

  let xmlStream = new XmlStream(fileStream);

  let counter = 0;

  xmlStream.on('endElement: Item', function(el) {
    liner.write(JSON.stringify(el));
    counter += 1;
  });

  xmlStream.on('end', function() {
    console.log(counter);
    liner.end();
  });

  return liner;
}