two classes reference each other javascript code example

Example 1: mixin in javascript mdn

let calculatorMixin = Base => class extends Base {
  calc() { }
};

let randomizerMixin = Base => class extends Base {
  randomize() { }
};

Example 2: mixin in javascript mdn

class Foo { }
class Bar extends calculatorMixin(randomizerMixin(Foo)) { }