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Generics are the polar opposites of automatic casting. They exist so you don't have to shoehorn one argument type into another.

JS isn't exactly the best environment to understand this. Ideally, use a compiler language that can show you the generated assembly (`gcc -S ...` with C++ for instance). Then you can see first-hand how type-specific implementations of a generic function look like.

Since when are generics automatic casting?

it's not but it's essentially the same practically speaking -- you get type A|B|C you write code to interact with it in just one way. Either by casting, or generics. weakly typed language do that for you, but they are looked down on.

I think you're missing the key point of statically typed languages, that the compiler catches errors for you. Generics are valuable because it allows users to write "generic" reusable functions that are still checked by the compiler. Of course dynamically-typed languages can do this easily, but that's not the point.

Blub 101