If a language can't detect the difference between a function a => f(a) and a function (a,b) => f(a,b) then I would consider it very weakly typed indeed. Dynamic

If a language can't detect the difference between a function a => f(a) and a function (a,b) => f(a,b) then I would consider it very weakly typed indeed. Dynamic vs static isn't really the issue, Python will also complain if you call a function with the wrong number of arguments. Then again I suppose that you could recreate JavaScripts behaviour even in the fairly strongly typed C# if you wanted to, by defining two overloaded versions of Map as follows: IEnumerable<B> Map<A,B>(this IEnumerable<B> list, Func<A,B> f) IEnumerable<B> Map<A,B>(this IEnumerable<B> list, Func<A,int,B> f) in that case C# would also choose the latter Map when you call it on a function with an optional int argument, similar to what JavaScript would do. However C# will never ignore an optional argument when you're passing around functions, so it's more consistent in that sense. And overloading a function in this way is not the best idea in the first place, it's just that JavaScript kind of encourages it.