index.js
function wrappedMapper (input, number, callback) {
    return mapper.call(null, input, function(err, data){
      callback(err, data, number)
    })
  }
function wrappedMapper (input, number, callback) {
    return mapper.call(null, input, function(err, data){
      callback(err, data, number)
    })
  }
function wrappedMapper ( input , number , callback ) { return mapper . call ( null , input , function ( err , data ) { callback ( err , data , number ) } ) }
Wrap the mapper function by calling its callback with the order number of the item in the stream.
Wrap the mapper function by calling its callback with the order number of the item in the stream .