It took a few years, but I finally understand why member functions of classes in JavaScript aren't automatically bound to their objects, in particular when used in callback arguments.
In most object-oriented languages, functions are members of a class--they exist in memory only once, and when they are called, this is
simply a hidden argument of the function which is assigned to the object that's calling it. Python makes this explicit by requiring that
the first argument of a class's member function be self (Python's equivalent of this--you can name self whatever you want, but
self is the convention).
class MyClass: