mixin.js

/**
 * Delegation based mixins for JavaScript.
 *
 * Non-specific extensions are used:
 *    __proto__
 *    __noSuchMethod__
 *
 * Tested in SpiderMonkey.
 *
 * Opened issues:
 *    - a mixin inherits from
 *      Object.prototype; a method may be
 *      found first there. It's may be avoided
 *      by setting __proto__ of a mixin
 *      to null, but then it's not possible
 *      to mixin other module to the mixin itself
 *    - reading inherited properties (not methods)
 *      of an object from the tailed prototype
 *    - ...
 *
 * @todo implement using Proxy objects and WeakMaps without
 * non-specific extensions.
 *
 * @author Dmitry A. Soshnikov <dmitry.soshnikov@gmail.com>
 * (C) Mit Style License
 */


/**
 * mixins a module to an object
 * @param {Object} module
 * @param {Object} to
 */
function mixin(module, to) {

  // convert to object if needed
  // or create a new one
  to = Object(to);

  // real prototype of an object
  var tailProto = to.__proto__ || Object.prototype;

  // a proxy object to inject as a
  // direct prototype of an object
  var proxy = {
    // first a mixed module is considered
    // for searching a method
    __proto__: module,
    // and then the tail of the prototype chain
    __noSuchMethod__: function (name, args) {
      return tailProto[name].apply(to, args);
    }
  };

  // inject the mixin module via proxy
  to.__proto__ = proxy;

  // proxied object
  return to;
}

// a mixin module
var m = {
  foo: function () {
    alert(['m.foo', this === o]);
  },
  baz: function () {
    alert(['m.baz', this === o]);
  }
};

var o = mixin(m, {});

o.foo(); // "m.foo", true
alert(o.foo === m.foo); // true, delegation is used

// another mixin
var m2 = {
  foo: function () {
    alert(['m2.foo', this === o]);
  },
  bar: function () {
    alert(['m2.bar', this === o]);
  }
};

// mixin it too
mixin(m2, o);

o.foo(); // shadows "m", now "m2.foo", true
o.bar(); // "m2.bar", true
o.baz(); // "m.baz", from previous module "m"

// third mixin module
var m3 = {
  test: function () {
    alert(['m3.test', this === o, this === m2]);
  }
};

// mixin it to another module
mixin(m3, m2);

o.test(); // "m3.test", true, false
m2.test(); // "m3.test", false, true

delete m2.foo; // remove shadowing "foo"
o.foo(); // again is taken from "m", "m.foo"