It is very simple to add methods to classes such as String prototypically. However, this affects all strings in the global scope.
How can I add a method accessible from all string literals like such: "foo".bar(), such that that method is only accessible within the scope of an ES6 class, say, or a function call?
I am afraid that you may not be able to do that. Also it seems like it's a bad idea.
It will be probably better if you just use normal function. Like: bar("foo")