Factory

Let's assume that we have a User class with two private variables: lastName:string and firstName:string. In addition, this simple class proposes that the hello method 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. It will more than likely 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]; 

Up until now, the TypeScript compiler hasn't complained, and it executes...