To update stream on middle using node js

streamMiddle.js

'use strict';
//Reference: https://www.safaribooksonline.com/blog/2013/05/01/using-streams-in-node-js/
var fs = require('fs');
var stream = require('stream');
//var Transform = require('stream').Transform;
var mx = {};
mx.bind_helo = '--------sham-----';


var source = fs.createReadStream('/home/sarv/node_modules/smail/s.txt');
var target = fs.createWriteStream('/home/sarv/node_modules/smail/replaced.txt');

function createFile() {
    var replaceSig = new stream.Transform({decodeStrings: false});
    replaceSig._transform = function (chunk, encoding, done) {
        done(null, chunk.toString().replace('by sham', 'by ' + mx.bind_helo));
    };
    source.pipe(replaceSig).pipe(target);
}
createFile()