I', having a Problem with the TypeScript compiler. Why aren't any errors shown for the following code snippet?
class A {
}
class B {
myVar:string;
}
class Foo {
bar(a:A){}
doSomething(){
this.bar(new B());
}
}
Since B is no A, shouldn't the bar-call with a B be prohibited?
The *bar-*call shouldn't be prohibited here, because TypeScript has structural type relationships. So, A and B are compared not just by names, but rather by enumerating and comparing their members, according to the specification. As follows.
bar() call requires its argument type to be assignable to the corresponding parameter type. So, "new B()" type should be assignable to "A" (specification section 4.12.1 "Overload resolution")
If we compare "B" and "A" structurally according to the algorithm presented in specification section 3.8.4 "Assignment compatibility", we see that "B" is assignable to "A".
If you want to experiment further, just add some property "x" to the class A. And you'll see your error.
