Use Javascript `in` operator for automatic type inference in TS

automaticTypeInterfence.ts

interface Admin {
  id: string;
  role: string;
}

interface User {
  email: string;
}

function redirect(usr: Admin | User) {
// input can only be of type "Admin" or "User". If it passes that if condition, that means it has the "role" property on it.
// The only type that includes the "role" property is admin, so it must be admin
  if ("role" in usr) {
    routeToAdminPage(usr.role);
  }
  else {
    routeToHomePage(usr.email);
  }
}