This is to get the unknown data (from json) and parse in go program for further opertions

go_interface_json_unmarsall.go

package main

// Importing libraries
import (
	"encoding/json"
	"fmt"
)

func main() {
  // Sample JSON string
	s := `{"url": "http://static01.nyt.com/images/2015/12/27/nyregion/27CHRISTIE1/27CHRISTIE1-thumbStandard.jpg",
          "format": "Standard Thumbnail",
          "height": 75,
          "width": 75,
          "type": "image",
          "subtype": "photo",
          "caption": "Gov. Chris Christie of New Jersey spoke about the Sept. 11, 2001, attacks at a Republican conference last month.",
          "copyright": "Stephen Crowley/The New York Times"
        }`
  
  // interface variable
	var someStruct interface{}
  
  // Unmarshalling the JSON into interface
	err := json.Unmarshal([]byte(s), &someStruct)
	if err != nil {
		fmt.Println("err")
	}
  
	// Converting into pure map (type asserting)
 	someStructMap, ok := someStruct.(map[string]interface{})
	if ok != true {
    	fmt.Println("Map coneversion is unsuccessfull")
	}
  
  // Printing the desired key value pair.
	fmt.Println(someStructMap["caption"])
	
	var someNum float64
	// type asserting the interface as float64
	someInt = somestructMap["height"].(float64)
	fmt.Println(someNum)
}

References:
[1] go - Getting invalid operation: mymap["title"] (type interface {} does not support indexing) when trying to index a map - Stack Overflow (https://stackoverflow.com/questions/25214036/getting-invalid-operation-mymaptitle-type-interface-does-not-support-in)

if there is a array in the json then use
someJSONArray, ok := someJSON.([]interface{})

Example:
https://play.golang.org/p/BHEYbsKAcpP