Factory pattern

Let's assume that we have a User class with two private variables: lastName:string and firstName:string. Also, this simple class proposes the method hello that prints "Hi I am", this.firstName, this.lastName:

class User{
     constructor(private lastName:string, private firstName:string){
     }
     hello(){
         console.log("Hi I am", this.firstName, this.lastName);
     }
 }

Now, consider that we receive users through a JSON API. Most likely, it will look something like this:

[{"lastName":"Nayrolles","firstName":"Mathieu"}...].  

With the following snippet, we can create a User:

let userFromJSONAPI: User = JSON.parse('[{"lastName":"Nayrolles","firstName":"Mathieu"}]')[0]; 

Until now; the TypeScript compiler doesn't complain, and it executes...