Golang : Fix type interface{} has no field or no methods and type assertions example

For new comers to Golang exploring interface{} for the first time or if you ever use interface{} to handle some arbitrary data, chances are that you will see this error message :

type interface {} has no field or method

or

type interface {} is interface with no methods

To fix this problem, you will need to convert the interface{} to a suitable type or extract the value(variable) with the explicit(correct)type from the interface{} with type assertions.

For example :

 // convert the json objects from interface{} to []interface{} 
 // [] = slice/array type

 var jsonObjs interface{}
 json.Unmarshal([]byte(jsonStr), &jsonObjs)
 objSlice, ok := jsonObjs.([]interface{})

or

 // convert invoices to map[string]interface{} 
 for index, value := range invoices.(map[string]interface{}) {
 fmt.Println(index, value)
 }

Hope this helps!

References :

https://www.socketloop.com/tutorials/golang-count-json-objects-and-convert-to-slice-array

http://golang.org/doc/effectivego.html#interfaceconversions