[DangitBobby]

Why was the `dogs` array initialized as an `Animal[]` type instead of `Dog[]` type which would forbid the addition of a `Cat` type?

Why would you be able to map a call to `woof` over an `Animal[]` when `Animal` doesn't implement `woof`? I don't understand how the SO link answers these questions.

[c-hendricks]

> Why was the `dogs` array initialized as an `Animal[]` type instead of `Dog[]`

That might be your confusion: it wasn't. Its type is `Dog[]`.

> which would forbid the addition of a `Cat` type?

Why would that be forbidden? The problematic method is `append_animals`, which only cares that both arguments satisfy `Animal`, which both `Dog` and `Cat` do.

> Why would you be able to map a call to `woof` over an `Animal[]` when `Animal` doesn't implement `woof`

Back to your root confusion, since for all intents and purposes, `dogs.map` thinks it's an array of dogs, it doesn't complain.

If `append_animals` was written like this, things would be fine:

    let append_animals = <T extends Animal>(animals: T[], animal: T) => animals.push(animal)

[DangitBobby]

I see what you're saying. Thanks for taking the time to explain.