code: function Mapper(mapper, flusher) {
    Transform.call(this, {
      objectMode: true,
      // Patch from 0.11.7
      // https://github.com/joyent/node/commit/ba72570eae938957d10494be28eac28ed75d256f
      highWaterMark: 16
    })
  
    this.mapper = mapper
    this.flusher = flusher
  }
docs: Transform stream that applies a mapper function to each chunk and pushes
  the mapped result.
  @param {Function} mapper
  @param {Function=} flusher