You still have interfaces. When you invoke a method on an interface, you don't know exactly which code will be called. It could do anything from crash to format your hard drive. In theory, the semantics of the interface should be well-specified and any implementation that doesn't conform to them is a bug, and Go culture holds pretty strictly to this, but there's nothing in the compiler that imposes solid restrictions on what the implementation of an interface can do.

If you're disciplined, you can write solid, easy-to-understand code in any language, Javascript included. I've worked on million+ line Javascript codebases and been productive (as a developer new to the codebase) within 3 days. But this imposes trade-offs in verbosity and flexibility that are often not worth it for most (non-Google-scale) JS codebases.