interface Person {
  "Id": number
  "First Name": string,
  "Last Name": string,
}

var person : Person = { Id: 5, "First Name": "Lorem", "Last Name": "Ipsum" };
var firstName = person["First Name"]; //this is neither validated by compiler nor I get auto completion for the properties
var _undefined = person["undefined"]; //compiler does not complain.

person["First Name"] = "Lorem"; //this is neither validated by compiler nor I get auto completion for the properties
person["undefined"] = "Ipsum"; //compiler does not complain.

interface Person {
  [key: "First Name" | "Last Name"]: string;
}