Typescript type checking mechanism

In typescript, the following compiles.

class Person {
  name : string;
}

class Employee {
  name : string;
}

var person : Person = new Employee();

Why does that work?? This does not:

class Person {
  name : string;
  surname : string;
}

class Employee {
  name : string;
}

var person : Person = new Employee();

With the exception that:

Type employee is not assignable to type Person, the property surname is missing in type Employee

So. Types are compared at compile time using property names??!

Edit: Yep. This is the code from the typescript compiler doing the checking.

            for (var i = 0; i < targets.length; i++) {
                var related = isRelatedTo(sources[i], targets[i], reportErrors);
                if (!related) {
                    return 0;
                }
                result &= related;
            }
            return result;