for var key in object of interface should be keyof type

[greg-hornby-roam]

TypeScript Version: 2.2.2

Code

interface MyInterface {
  a: number;
  b: string;
  c: boolean;
}

var myObj: MyInterface = {a: 5, b: "foo", c: true};

for (var key in myObj) {
    var getValue = myObj[key];
}

Expected behavior:
The var key should have type keyof typeof myObj evaluating to 'a' | 'b' | 'c' instead of just string. This would then allow the use of myObj[key] without having to cast to any
ie (myObj as any)[key]

[DanielRosenwasser]

Unfortunately this wouldn't be appropriate behavior - if you have a subtype of MyInterface (i.e. a type that's more specific than MyInterface), then the keys of myObj actually grow, which gives you a less specific type.

So you might think that at runtime your keys are a, b, and c, but in reality you might also have a d, e, and f property as well.

See #12892 for a similar discussion.