Essentially, we are using more type annotations then the style guide recommends. Like Sergey says, we're deviating from the style guide in a place where the guide is more a recommendation then a rule. For public API types are strongly recommended. Within a method, they are optional. I prefer using them within a method not for any runtime checking we might get, but because the types help aid understanding of the code for people reading it. When casually scanning the code, it's nice to know whether a variable is a String, File, or List. I do omit types when its very clear what the type is, or if the code using it is very short. I also dramatically shorten var names when I'm trying to be very compact:

`return fileList.map((`__f__`) => f.name).join(',');`