How to organize such a stream?

Question

Node.js

Hello. There is a code:

let spawn = require('child_process').spawn,
  fs    = require('fs'),
  stream = require('stream').Writable;

let a = new stream();
a._write = (chunk, encoding, done) => {

}

function test () {
  this.ls = spawn('Некая команда');
  return this.ls.stdin;
}

fs.createReadStream('test.file')
  .pipe(test())
  .pipe(a);

A certain process spawns, which receives data on stdin and should give it on stdout. So here's how to get data from the stdout of the process in this case and transfer it to the writable stream (variable a)