There's nothing wrong w/ polymorphic functions, but there has to be a reasonable rule to apply for what kinds of shapes of data are permitted. Trying to aggressively handle unforeseen cases instead of indicating an error doesn't actually help anyone, it's akin to swallowing exceptions. It's why falsey values and automatic type conversion are some of the most problematic features in javascript.

Your example of how to handle max for array arguments is exactly the kind of thing I would discourage in js code, because it overloads the meaning of "maximum" to data that don't have a natural way of assigning an ordering if heterogeneous arguments are permitted (actually it sounds like you're saying something like max([1], [9]) should return "9", which doesn't correspond to an intuitive understanding of "maximum" at all given it returns something that isn't either of the passed arguments). Whereas the rust code can use the "PartialOrd" trait to ensure we only try to produce a maximum of like elements with a defined ordering relation. That's fine for a limited number of cases where its easy to show type equivalence in js, but I think it's problematic as a general design principle, and because you don't have ways to express the limits of polymorphism I've seen this kind of thing proliferate to unsafe uses of a function very quickly.