what is it

io.Reader essentially reads the byte array ([]byte).

type Reader interface {
	Read(p []byte) (n int, err error)
}

Create new buffer/byte array

bytes.Buffer is used when you need a writer that writes to memory.

import (
  "bytes"
)

func main() []byte {
  // create buffer
  buf := new(bytes.Buffer)

  // run encoding on the `buf`
  err := jpeg.Encode(buf, new_image, nil)

  // convert buffer to reader
  reader := bytes.NewReader(buf.Bytes())

  return buf.Bytes()
}