How to do runtime type casting in TypeScript?

Question

Currently I'm working on a typescript project and I'm really enjoying the type inference that TypeScript brings to the table. However - when getting objects from HTTP calls - I can cast them to the desired type, get code completion and call functions on them compile time, but those result in errors runtime

Example:

class Person{
    name: string;

    public giveName() {
        return this.name;
    }

    constructor(json: any) {
        this.name = json.name;
    }
}

var somejson = { 'name' : 'John' }; // Typically from AJAX call
var john = <Person>(somejson);      // The cast

console.log(john.name);       // 'John'
console.log(john.giveName()); // 'undefined is not a function'

Although this compiles nicely - and intellisense suggests me to use the function, it gives a runtime exception. A solution for this could be:

var somejson = { 'name' : 'Ann' };
var ann = new Person(somejson);

console.log(ann.name);        // 'Ann'
console.log(ann.giveName());  // 'Ann'

But that will require me to create constructors for all my types. In paticular, when dealing with tree-like types and/or with collections coming in from the AJAX-call, one would have to loop through all the items and new-up an instance for each.

So my question: is there a more elegant way to do this? That is, cast to a type and have the prototypical functions available for it immediately?

Answer 1

The prototype of the class can be dynamically affected to the object:

function cast<T>(obj: any, cl: { new(...args): T }): T {
  obj.__proto__ = cl.prototype;
  return obj;
}

var john = cast(/* somejson */, Person);

See the documentation of __proto__ here.