golang how to define optional string code example

Example: optional parameter golang

package main

import (
	"fmt"
)

func Testing(value ...interface{}) {
  for _, v:= range value {
  	fmt.Println(v)
  }
}

func main() {

	Testing("Content-Type", "application/json")
	Testing("200")
	fmt.Println("Hello, playground")
}