Calling JSON decoder repetitvely because of small change in response structure

I have added Github and Google authentication system to my web application. I am looking to get user email in both of these cases. I tried to make a one function which would make the API request and get the email.<br>
I ran into an issue when Google returned a JSON object and Github a JSON array as a response. <br>
I cant figure out a way how I could avoid calling the JSON decoder twice as I cant have a same type variable for both of them.

// Sends a request to the API and
// authorizes it by setting HTTP header "Authorization" to authHeader value
func getUserEmail(endpoint, authHeader, provider string) (string, error) {
var email string       // Store user email here
var client http.Client // Create client so we can modify request headers
// Create a GET request to the endpoint
req, err := http.NewRequest("GET", endpoint, nil)
if err != nil {
return "", err
}
// Authorize the request by using the HTTP header
req.Header.Add("Authorization", authHeader)
// Give the data back as JSON
req.Header.Add("accept", "application/json")
// Send the request
resp, err := client.Do(req)
if err != nil {
fmt.Println("Internet connection or client policy error")
return "", err
}
defer resp.Body.Close()
if provider == "google" {
var response map[string]interface{}
err = json.NewDecoder(resp.Body).Decode(&response)
if err != nil {
fmt.Println("Error occured during decoding access token response")
return "", err
}
email = response["email"].(string)
} else if provider == "github" {
var response []map[string]interface{}
err = json.NewDecoder(resp.Body).Decode(&response)
if err != nil {
fmt.Println("Error occured during decoding access token response")
return "", err
}
email = response[0]["email"].(string)
} else {
return "", errors.New("invalid provider")
}
return email, nil
}

You can unmarshal into var response interface{}. Once the json is unmarshaled you can do a type assertion on the response to check if it's []interface{} or map[string]interface{} and go from there.

var email string
var response interface{}
if err := json.NewDecoder(r.Body).Decode(&response); err != nil {
    return err
}

// If you are sure that the structure of the content of the response,
// given its type, is always what you expect it to be, you can use a
// quick-and-dirty type switch/assertion.
switch v := response.(type) {
case []interface{}:
    email = v[0].(map[string]interface{})["email"].(string)
case map[string]interface{}:
    email = v["email"].(string)
}

// But! If you're not sure, if the APIs don't provide a guarantee,
// then you should guard against panics using the comma-ok idiom
// at every step.
if s, ok := response.([]interface{}); ok && len(s) > 0 {
    if m, ok := s[0].(map[string]interface{}); ok && len(m) > 0 {
        email, _ = m["email"].(string)
    }
} else if m, ok := response.(map[string]interface{}); ok && len(m) > 0 {
    email, _ = m["email"].(string)
}

You can also pre-allocate a pointer to the expected type based on the provider value and unmarshal the request body into that, this will reduce the number of type assertions necessary but will require pointer-dereferencing.

var email string
var response interface{}
if provider == "google" {
    response = new(map[string]interface{})
} else if provider == "github" {
    response = new([]map[string]interface{})
}
if err := json.NewDecoder(r.Body).Decode(response); err != nil {
    return err
}

switch v := response.(type) {
case *[]map[string]interface{}:
    email = (*v)[0]["email"].(string) // no need to assert the slice element's type
case *map[string]interface{}:
    email = (*v)["email"].(string)
}