Decoding JSON int into string

You can use the type json.Number which is implemented as a string:

type User struct {
        Id       json.Number `json:"user_id"`
        Username string      `json:"user_name"`
}

Then you can simply convert it in any other code:

stringNumber := string(userInstance.Id)

Playground: https://play.golang.org/p/2BTtWKkt8ai