<NME> mixwith.js
<BEF> 'use strict';

// used by apply() and isApplicationOf()
const _appliedMixin = '__mixwith_appliedMixin';

/**
 * A function that returns a subclass of its argument.
 *
 * @example
 * const M = (superclass) => class extends superclass {
 *   getMessage() {
 *     return "Hello";
 *   }
 * }
 *
 * @typedef {Function} MixinFunction
 * @param {Function} superclass
 * @return {Function} A subclass of `superclass`
 */

/**
 * Applies `mixin` to `superclass`.
 *
 * `apply` stores a reference from the mixin application to the unwrapped mixin
 * to make `isApplicationOf` and `hasMixin` work.
 *
 * This function is usefull for mixin wrappers that want to automatically enable
 * {@link hasMixin} support.
 *
 * @example
 * const Applier = (mixin) => wrap(mixin, (superclass) => apply(superclass, mixin));
 *
 * // M now works with `hasMixin` and `isApplicationOf`
 * const M = Applier((superclass) => class extends superclass {});
 *
 * class C extends M(Object) {}
 * let i = new C();
 * hasMixin(i, M); // true
 *
 * @function
 * @param {Function} superclass A class or constructor function
 * @param {MixinFunction} mixin The mixin to apply
 * @return {Function} A subclass of `superclass` produced by `mixin`
 */
export const apply = (superclass, mixin) => {
  let application = mixin(superclass);
  application.prototype[_appliedMixin] = unwrap(mixin);
  return application;
};

/**
 * Returns `true` iff `proto` is a prototype created by the application of
 * `mixin` to a superclass.
 *
 * `isApplicationOf` works by checking that `proto` has a reference to `mixin`
 * as created by `apply`.
 *
 * @function
 * @param {Object} proto A prototype object created by {@link apply}.
 * @param {MixinFunction} mixin A mixin function used with {@link apply}.
 * @return {boolean} whether `proto` is a prototype created by the application of
 * `mixin` to a superclass
 */
export const isApplicationOf = (proto, mixin) =>
  proto.hasOwnProperty(_appliedMixin) && proto[_appliedMixin] === unwrap(mixin);

/**
 * Returns `true` iff `o` has an application of `mixin` on its prototype
 * chain.
 *
 * @function
 * @param {Object} o An object
 * @param {MixinFunction} mixin A mixin applied with {@link apply}
 * @return {boolean} whether `o` has an application of `mixin` on its prototype
 * chain
 */
export const hasMixin = (o, mixin) => {
  while (o != null) {
    if (isApplicationOf(o, mixin)) return true;
    o = Object.getPrototypeOf(o);
  }
  return false;
}


// used by wrap() and unwrap()
const _wrappedMixin = '__mixwith_wrappedMixin';

/**
 * Sets up the function `mixin` to be wrapped by the function `wrapper`, while
 * allowing properties on `mixin` to be available via `wrapper`, and allowing
 * `wrapper` to be unwrapped to get to the original function.
 *
 * `wrap` does two things:
 *   1. Sets the prototype of `mixin` to `wrapper` so that properties set on
 *      `mixin` inherited by `wrapper`.
 *   2. Sets a special property on `mixin` that points back to `mixin` so that
 *      it can be retreived from `wrapper`
 *
 * @function
 * @param {MixinFunction} mixin A mixin function
 * @param {MixinFunction} wrapper A function that wraps {@link mixin}
 * @return {MixinFunction} `wrapper`
 */
export const wrap = (mixin, wrapper) => {
  Object.setPrototypeOf(wrapper, mixin);
  if (!mixin[_wrappedMixin]) {
    mixin[_wrappedMixin] = mixin;
  }
  return wrapper;
};

/**
 * Unwraps the function `wrapper` to return the original function wrapped by
 * one or more calls to `wrap`. Returns `wrapper` if it's not a wrapped
 * function.
 *
 * @function
 * @param {MixinFunction} wrapper A wrapped mixin produced by {@link wrap}
 * @return {MixinFunction} The originally wrapped mixin
 */
export const unwrap = (wrapper) => wrapper[_wrappedMixin] || wrapper;

const _cachedApplications = '__mixwith_cachedApplications';

/**
 * Decorates `mixin` so that it caches its applications. When applied multiple
 * times to the same superclass, `mixin` will only create one subclass, memoize
 * it and return it for each application.
 *
 * Note: If `mixin` somehow stores properties its classes constructor (static
 * properties), or on its classes prototype, it will be shared across all
 * applications of `mixin` to a super class. It's reccomended that `mixin` only
 * access instance state.
 *
 * @function
 * @param {MixinFunction} mixin The mixin to wrap with caching behavior
 * @return {MixinFunction} a new mixin function
 */
export const Cached = (mixin) => wrap(mixin, (superclass) => {
  // Get or create a symbol used to look up a previous application of mixin
  // to the class. This symbol is unique per mixin definition, so a class will have N
  // applicationRefs if it has had N mixins applied to it. A mixin will have
  // exactly one _cachedApplicationRef used to store its applications.

  let cachedApplications = superclass[_cachedApplications];
  if (!cachedApplications) {
    cachedApplications = superclass[_cachedApplications] = new Map();
  }

  let application = cachedApplications.get(mixin);
  if (!application) {
    application = mixin(superclass);
    cachedApplications.set(mixin, application);
  }

  return application;
});

/**
 * Decorates `mixin` so that it only applies if it's not already on the
 * prototype chain.
 *
 * @function
 * @param {MixinFunction} mixin The mixin to wrap with deduplication behavior
 * @return {MixinFunction} a new mixin function
 */
export const DeDupe = (mixin) => wrap(mixin, (superclass) =>
    (hasMixin(superclass.prototype, mixin))
      ? superclass
      : mixin(superclass));

/**
 * Adds [Symbol.hasInstance] (ES2015 custom instanceof support) to `mixin`.
 *
 * @function
 * @param {MixinFunction} mixin The mixin to add [Symbol.hasInstance] to
 * @return {MixinFunction} the given mixin function
 */
export const HasInstance = (mixin) => {
  if (Symbol && Symbol.hasInstance && !mixin[Symbol.hasInstance]) {
    Object.defineProperty(mixin, Symbol.hasInstance, {
      value(o) {
        return hasMixin(o, mixin);
      },
    });
  }
  return mixin;
};

/**
 * A basic mixin decorator that applies the mixin with {@link apply} so that it
 * can be used with {@link isApplicationOf}, {@link hasMixin} and the other
 * mixin decorator functions.
 *
 * @function
 * @param {MixinFunction} mixin The mixin to wrap
 * @return {MixinFunction} a new mixin function
 */
export const BareMixin = (mixin) => wrap(mixin, (s) => apply(s, mixin));

/**
 * Decorates a mixin function to add deduplication, application caching and
 * instanceof support.
 *
 * @function
 * @param {MixinFunction} mixin The mixin to wrap
 * @return {MixinFunction} a new mixin function
 */
export const Mixin = (mixin) => DeDupe(Cached(BareMixin(mixin)));

/**
 * A fluent interface to apply a list of mixins to a superclass.
 *
 * ```javascript
 * class X extends mix(Object).with(A, B, C) {}
 * ```
 *
 * The mixins are applied in order to the superclass, so the prototype chain
 * will be: X->C'->B'->A'->Object.
 *
 * This is purely a convenience function. The above example is equivalent to:
 *
 * ```javascript
 * class X extends C(B(A(Object))) {}
 * ```
 *
 * @function
 * @param {Function} [superclass=Object]
 * @return {MixinBuilder}
 */
export const mix = (superclass) => new MixinBuilder(superclass);

class MixinBuilder {

  constructor(superclass) {
    this.superclass = superclass || Object;
  }

  /**
   * Applies `mixins` in order to the superclass given to `mix()`.
   *
   * @param {Array.<Mixin>} mixins
   * @return {Function} a subclass of `superclass` with `mixins` applied
   */
  with(...mixins) {
    return mixins.reduce((c, m) => m(c), this.superclass);
  }
}

<MSG> Omitting superclass in mix() extends an empty class, not Object.
<DFF> @@ -234,7 +234,7 @@ export const mix = (superclass) => new MixinBuilder(superclass);
 class MixinBuilder {
 
   constructor(superclass) {
-    this.superclass = superclass || Object;
+    this.superclass = superclass || class {};
   }
 
   /**

Omitting superclass in mix() extends an empty class, not Object.