Read up about javascript modules

[surfmuggle]

A module is a piece of program that specifies which other pieces it relies on and which functionality it provides for other modules to use (its interface). source Eloquent JavaScript - Modules

[surfmuggle]

Without modules a function expression that is immediately invoked can be used:

const weekDay = function() {
  const names = ["Sunday", "Monday", "Tuesday", "Wednesday",
                 "Thursday", "Friday", "Saturday"];
  return {
    name(number) { return names[number]; },
    number(name) { return names.indexOf(name); }
  };
}();

Example how to use it console.log(weekDay.name(weekDay.number("Sunday")));. The const weekDay has two functions name(number) and number(name).