Private method/property with WeakMap

private_instance_weakmap.js

'use strict';

/**
 * Demonstration for creating private variable/method in JavaScript
 * constrcutors, with WeakMap trick.
 *
 * The idea here is to create two WeakMaps to link a public instance and
 * a private instance, and two shorthand query function to query between two.
 * 
 * Both instances can therefore have their own methods respectively.
 * An "apply()" apply trick is also demonstrated in privateMethod2() but
 * it's too confusing thus not recommended. 
 * "pubThis" and "privThis" should be more understandable in this setup.
 *
 * Note that even though this is shown passing "window" as exporting object,
 * it can surely be the module.exports in CommonJS environment.
 * The point here is to have your own closure to encapsulate the WeakMaps.
 * 
 * I expect, if anyone will ever use this setup, most of the methods will be
 * in the PrivateClass and PublicClass will simply be a wrapper/pointers
 * to the methods & properties. It would be easier to keep most of the lines
 * pointing to the same "this".
 */
(function(exports) {
  var publicPrivateInstanceMap = new WeakMap();
  var _p = publicPrivateInstanceMap.get.bind(publicPrivateInstanceMap);

  var privatePublicInstanceMap = new WeakMap();
  var _rp = privatePublicInstanceMap.get.bind(privatePublicInstanceMap);

  var PrivateClass = function() {
  };

  PrivateClass.prototype.linkWithPublicInstance = function(publicInstance) {
    publicPrivateInstanceMap.set(publicInstance, this);
    privatePublicInstanceMap.set(this, publicInstance);
  };

  PrivateClass.prototype.privateProperty = 'myPrivateProperty';

  PrivateClass.prototype.privateMethod = function() {
    var val = this.privateProperty;

    // Get pubic instance and set public property.
    var pubThis = _rp(this);
    pubThis.publicProperty = 'bar';
  };

  PrivateClass.prototype.privateMethod2 = function() {
    // Call ourselve with public instance instead.
    if (this instanceof PrivateClass) {
      return this.privateMethod.apply(_rp(this), arguments);
    }

    // "this" is is the public instance beyond this point.
    this.publicProperty = 'baz';
  };

  var PublicClass = function PublicClass() {
    var privateInstance = new PrivateClass();
    privateInstance.linkWithPublicInstance(this);
  };

  PublicClass.prototype.publicProperty = 'foo';

  PublicClass.prototype.publicMethod = function() {
    // Get private instance.
    var privThis = _p(this);
    privThis.privateProperty = 'newPrivateValue';

    // Call private method.
    privThis.privateMethod();
  };

  // Only export the PublicClass
  exports.PublicClass = PublicClass;
})(window);

we use a similar way in https://github.com/mozilla-b2g/gaia/blob/793773bb2944b42a85dd160049e605cbd880c4da/apps/sms/js/subject_composer.js#L96-L111 :)

@julienw woot!