TypeScript interface type checking from the beginning tutorial - interface

Question

I've just started learning TypeScript and I'm having a hard time wrapping my head around an interface behavior from the beginning tutorial on http://www.typescriptlang.org/Playground/#tut=ex5
As I understand it, the interface should enforce the type of the parameters, however this particular line throws me off
var user = new Student("Jane", "M.", "User");
It compiles correctly and all is well, but when I modify it to
var user = new Student(1, 2, 3);
it also compiles just fine.
Could anyone elaborate why it's working ?
I understand that this is a beginners question but I could not find any info on this searching online and I don't have any TypeScript experts around me.
Thanks in advance,
Eugene

Answer 1

The type of the Student constructor parameters is any because there is no type annotation:
class Student {
fullname : string;
constructor(public firstname, public middleinitial, public lastname) {
this.fullname = firstname + " " + middleinitial + " " + lastname;
}
}
If we change it to have some type annotations, we'll get an error:
class Student {
fullname : string;
constructor(public firstname: string, // <-- add ': string' here
public middleinitial: string, // and here
public lastname: string) { // and here
this.fullname = firstname + " " + middleinitial + " " + lastname;
}
}
var x = new Student(1, 2, 3); // Error