How can I cast a JSON object to a TypeScript class instance without any external library?

I am trying to cast a JSON object like the following example:

class Person {
    constructor(
      public firstName, 
      public lastName
   ) {}
   function getName() {
      return this.firstName + “ “ + this.lastName;
   }
}

The goal is to parse the JSON which looks like this:

    {
    firstName: “Max“,
    lastName: “Mustermann“
    }

to an instance of the above class to access all properties and methods.

Is there an easy way to create a function which can make such a functionality possible for such type of JSONs/classes?
Also nested objects should be possible.

I can write a factory method for every different class but there should be an better way to get such functionality.

Use Object.assign

Create your class like this, observe the constructor

class Person {
       public firstName: string; 
       public lastName: string;
       public constructor(init?: Partial<Person>) {
        Object.assign(this, init);
       }
      function getName() {
        return this.firstName + “ “ + this.lastName;
      }
   }

Pass the json into this constructor

let person =  new Person({firstName: "A", lastName: "B" });
console.log(person);