The result add(1: Int, "2": String) -> "12": String can be achieved in any language, regardless of how strict the type system is. All your example shows is that

The result add(1: Int, "2": String) -> "12": String can be achieved in any language, regardless of how strict the type system is. All your example shows is that in elxir no such `add(int, string)` operation is provided as part of the standard library/environment, while in javascript, it is. The "+" (operator) case is only special in languages that do not implement operator overloading -- it is completely orthogonal to the type system.