What would "defining an interface" even mean in a non-type checked language? I'd say: ECMAScript already has better interface support than Java has because it supports implicit interfaces. In Java two classes can only be interface-compatible if they have a shared dependency/are at least indirectly coupled. In a language with duck typing and without static typing interfaces are present implicitly.
You're correct that adding interfaces to JavaScript would also require adding optional type declarations. But there are lots of languages based on JavaScript, or which compile to JavaScript, which offer optional type declarations. It seems to me they could have gone down this path.
I don't agree that it's better to have an "interface" which exists purely in the head of every programmer working on the code. Languages can be doing more for us, and it doesn't have to be the kind of verbosity that Java offers.
Maybe that was a misunderstanding: I didn't mean to say that interfaces should exist purely in the head of the programmer. I did say that implicit interfaces/protocols (Go, Objective-C) are better than interfaces that need shared dependencies (Java/pure virtuals in C++). If I had to order it I'd say: duck typing better than explicit interfaces; implicit interfaces that are backed by static type checks "better than" duck typing. The "better than" in quotes for the latter because it really is more about how much static analysis the language supports and it's more a matter of opinion.
But I'm relatively convinced that interfaces/protocols that require coupling of implementations are clearly inferior to the ones that are implicit.
Probably a lot like Clojure's protocols, which are precisely that. Effectively, functions declared in a protocol are just lookup tables that resolve to a particular implementation based on an object's type at runtime. You can query a value at runtime for whether it implements a protocol, too.
A statically typed language enforces an interface, but an interface is just a concept. All it does is ask: does this object respond to the following methods? You and your team can ask that question, or the type checker can.
