Copy streams with callbacks

copy.js

var copy = function(inputStream, outputStream, callback){
  var copyNextChunk = function(){
    inputStream.read(fuction(err, chunk){    
      if(err) return callback(err)
      // When chunk == null there's no data, copying is finished.
      if(!chunk) return callback()
      outputStream.write(chunk, function(err){
        // Callback called only when chunk of data 
        // delivered to the recipient and
        // we can send another one.
        if(err) return callback(err)
        copyNextChunk()
      })  
    })
  }
}