如何将变量值定义为变量？

Question

我想将对象值定义为新变量。我怎样才能做到这一点？ 这是一个例子：

var msg = '{
  "fname": {
    "req": "first name required"
  },
  "email": {
    "req": "email is required",
    "validate": "provide valid email"
  }
}';
var obj = JSON.parse(msg);

console.log(msg.fname.req) // logs "first name required", that's fine

这是我到目前为止所尝试的：

 new_fname = 'fname'
 console.log(msg.new_fname.req) // this throws an error ("req is not defined")

如何将new_fname值声明为新变量？