Make get_or_create_one function handle different numbers of callback arguments

// TODO: But in this case, the callback takes two arguments. How can
// we handle different numbers of callback arguments?

Maybe by making the callback arg be an object or list that is parsed. This seems to be the best solution. Or does JavaScript naturally ignore missing arguments? Or try parsing the argument list from inside the function, if JavaScript allows functions to be passed args that are not listed in the function definition.