ngs.js

var thr = require('through2');

module.exports = function(v){
  return thr(function(c,e,n){
    this.push(v(c));
    n();
  });
};

if(!module.parent){
  var fs = require('fs');
  var forJpg = function(buf){
    for(var i=0; i<buf.length; i++){
      if(buf[i]===18){
        if(Math.random()>0.5){
          buf[i] = Math.floor(Math.random()*10);
        }else{
          buf[i] = buf[i+1];
        }
      }
    }
    return new Buffer(buf)
  };

  fs.createReadStream('test.jpg')
    .pipe(module.exports(forJpg))
    .pipe(fs.createWriteStream('out.jpg'));
}