Javascript mixins when using the module pattern

Question

I've been using the module pattern for a while, but recently have started wanting to mix in functions and properties into them to increase code re-use. I've read some good resources on the subject, but still am a bit uncertain as to the best approach. Here is a module:

var myModule = function () {
    var privateConfigVar = "Private!";

    //"constructor"
    function module() {}

    module.publicMethod = function () {
        console.log('public');
    }

    function privateMethod1() {
        console.log('private');
    }

    return module;
}

And here is a mixin object:

var myMixin = function () {};
Mixin.prototype = {
    mixinMethod1: function () {
        console.log('mixin private 1');
    },
    mixinMethod2: function () {
        console.log('mixin private 2');
    }
};

Ideally, I'd like to mix-in some methods from other objects as private methods and some as public methods, so that I could call some "extend" function, with a param as "private"/"public". So, that

mixin(myModule, myMixin, "private");

makes the myMixin methods available within myModule by just calling mixinMethod1() and have correct scope, and:

mixin(myModule, myMixin, "public");

makes the myMixin methods available within myModule by calling module.mixinMethod1() and have correct scope

I've tried using a method that copies properties from one prototype to another, I've tried the underscore extend method to copy properties of the object from one to to the other, and various things in between. I think I'm a bit turned around regarding scope and prototypes at this point, and would love some direction as to how best to do mixins like this when using the module pattern. Note that it doesn't matter what the object myMixin looks like (whether adding functions to the prototype, or a module itself), I'm just trying to figure out some way to make it work.

Thank!

Answer 1

So that [some code] makes the myMixin methods available within myModule by just calling mixinMethod1() and have correct scope

That's impossible. You cannot modify a scope by calling a function, especially not from outside. See also Is it possible to import variables in JavaScript? for the design reasons of that.

So, what can you do?

From outside the module

Nothing to the private scope(s) of module functions. And you cannot use the private functions of the module, obviously. You can extend its prototype with methods (which is the most common), you can even decorate its constructor function. Within those, you can use your own private functions, either completely static ones or class-specific ones.

var myMixin = (function() {
    // everything class-unspecific but mixin-local
    var staticMixinVariables, …;
    function globalPrivateFunction(){…}
    function staticMethod(){…}

    return function(mod) {
        // everything class-specific
        // also using the locals from above
        mod.staticHelper = function() { staticMixinVariable … };
        mod.prototype.mixinMethod1 = staticMethod;
        mod.prototype.mixinMethod2 = function(){…};
        …
    };
})();

// Example:
myMixin(SomeClass)

From within the module

Using the mixin in the module code itself can allow for much greater flexibility.

var myMixin = (function() {
    // everything class-unspecific but mixin-local
    …
    return {
        publicHelper1: function(){…},
        publicHelper2: function(){…},
        decorateInstance: function(o) {
            o.xy = …;
        },
        extendPrototype: function(proto) {
            // everything class-specific
            // also using the locals from above
            proto.mixinMethod1 = staticMethod;
            proto.mixinMethod2 = function(){…};
            …
        }
    };
})();

With such an interface, it becomes easy to construct a class that is using this as a mixin (not inheriting from it):

var myClass = (function() {
    function Constructor() {
        myMixin.decorateInstance(this);
        …
    }
    Constructor.prototype.method1 = function() { myMixin.publicHelper1() … };
    Constructor.prototype.method2 = function() { … };
    myMixin.extendPrototype(Constructor.prototype);
    Constructor.myHelper = myMixin.publicHelper2; // re-export explicitly
    return Constructor;
})();

However, the mixin will never have access to the private class variables, nor can it present a private, class-specific API. Still, we can use dependency injection to provide that access explicitly (and having a mixin factory in effect):

var myClass = (function() {
    var … // private class functions and variables
    var mixer = myMixin(privateClassHelper,
                        privateClassVariable,
                        function setPrivateVar(x) {…},
                        … );
    var myHelper = mixer.customHelper, … // local "aliases"
    function Constructor(localX) {
        mixer.decorateInstance(this, localX);
        …
    }
    … // further using the class-specific private mixer
    return Constructor;
})();

---

Not all techniques shown above need to be used in every mixin, just choose the ones you need. Not all possible techniques are shown in the above examples, also :-) The mixin pattern can be applied onto a plain module or inside its declaration as well, the above examples have only shown classes with prototypes.

For a few good examples, and a theoretical distinction between (stateless) Traits, (stateful) Mixins and their "privileged" counterparts, have a look at this presentation.

Answer 2

The with keyword can be very usefull to define a scope, but it has also some drawbacks (it is by the way forbidden in strict mode).

Using the with keyword, you can define a private variable privateScope within the body of your module, that would contain all your provate methods :

var myModule = function () {

    var privateConfigVar = "Private!";
    var privateScope = {};

    //"constructor"
    function module() {}

    var proto = module.prototype;//avoids multiple attribute lookup

    //Let's re-define you example' private method, but with a new strategy
    privateScope['privateMethod1'] = function() {
        console.log('private');
    }

    proto.publicMethod = function () {
        with(privateScope){
            //this call should work
            privateMethod1();
        }
        console.log('public');
    }

    proto.publicMethod2=function(name,fn){
        with(privateScope){
            //this will be defined later by a Mixin
            otherPrivateMethod();
        }
        console.log('public2');
    }

    proto.definePrivateFunction=function(name,fn){
        privateScope[name] = fn;
    }



    return module;
}

Your mixin will use the definePrivateFunction we just defined to add private methods to the private scope :

//An example mixin implementation
function Mixin(source,target,flag){
    if(flag==="private"){
        for(var currentMethodName in source){
            target.definePrivateFunction(currentMethodName,source[currentMethod])
        }
    }else{
        for(var currentMethodName in source){
            target[currentMethodName]=source[currentMethod];
        }
    }
}

The following code should work fine:

var test = myModule();
var testInstance = new test();
testInstance.publicMethod();// will call the private method defined internally

Mixin({
          otherPrivateMethod:function(){
                        console.log("other Prvate Method called")
                      }
      },test.prototype,"private");

testInstance.publicMethod2();// will call the private method defined by the mixin

Answer 3

Ideally, I'd like to mix-in some methods from other objects as private methods and some as public methods, so that I could call some "extend" function, with a param as "private"/"public". ...

As it already has been mentioned, there is no way of achieving exactly this goal.

So, that ... makes the myMixin methods available within myModule by just calling mixinMethod1() and have correct scope, and: ... makes the myMixin methods available within myModule by calling module.mixinMethod1() and have correct scope.

And referring to scope ... this is a closed address space created by functions. Except for closures, scope only is available during a function's runtime within this function's body. It never ever can be manipulated/spoofed.

The term one is looking for is context. JavaScript, being in many ways highly dynamic, is build upon late binding (the object/target/context a method is called on gets evaluated/looked up at runtime) and two kinds of delegation. Context gets delegated either automatically by "walking the prototype chain" or explicitly by one of both call methods which every function object does provide - either call or apply.

Thus JavaScript already at language core level does offer a function based Mixin pattern that is mightier than any of the available extend(s) or mixin implementations for it provides delegation for free and is able of passing around state which almost every of the blamed helpers does lack unless there was effort of implementing this feature again in a rather roundabout fashion (or ass-backwards to put it bluntly).

Bergi for his explanation already earned the bounties. Within his answer's last paragraph there is a link to resources of mine that already got outdated 3 month after giving the referred talk. Due of not having enough reputation points, I'm not able to comment his answer directly. For this I'll take the chance pointing now to the latest state of my personal research and understanding of »The many talents of JavaScript for generalizing Role Oriented Programming approaches like Traits and Mixins«

Back again answering the OP's question.

I'm going to change the two first given code examples from the assumed module pattern and the rather exemplarily provided mixin code base towards a plain constructor function and what I'm meanwhile tempted to call a "proxified" and/or "bicontextual" mixin in order to boil down the mechanics of delegating two different target/context objects at once. Thus demonstrating a pure function based mixin pattern that might come closest to what the OP tries to achieve.

var MyBicontextualMixin = function (localProxy) {

  localProxy.proxifiedAccessible = function () {
    console.log("proxified accessible.");
  };
  this.publiclyAccessible = function () {
    console.log("publicly accessible.");
  };
};

var MyConstructor = function () {
  var localProxy = {};
  MyBicontextualMixin.call(this, localProxy);

  var locallyAccessible = localProxy.proxifiedAccessible;

  // call 'em
  locallyAccessible();        // "proxified accessible."
  this.publiclyAccessible();  // "publicly accessible."
};

(new MyConstructor);

// will log:
//
// proxified accessible.
// publicly accessible.

This special pattern also is the underlying base for composing pure function based Traits that rely on conflict resolution functionality provided by "proxified" Mixins that won't expose this functionality into public.

And for not ending up that theoretical there will be a "real world example", composing a Queue module out of various reusable mixins that entirely worship the approach of DRY. It also should answer the OP's question about how to achieve encapsulation and exposition build only upon the module pattern and function based mixin composition.

var Enumerable_first_last_item = (function (global) {

  var
    parseFloat = global.parseFloat,
    math_floor = global.Math.floor,

  // shared code.

    first = function () {
      return this[0];
    },
    last = function () {
      return this[this.length - 1];
    },
    item = function (idx) {
      return this[math_floor(parseFloat(idx, 10))];
    }
  ;

  return function () { // [Enumerable_first_last_item] Mixin.
    var enumerable = this;

    enumerable.first = first;
    enumerable.last = last;
    enumerable.item = item;
  };

}(window || this));



var Enumerable_first_last_item_proxified = function (list) {
  Enumerable_first_last_item.call(list);

// implementing the proxified / bicontextual [Enumerable_first_last_item] Mixin.
  var enumerable = this;

  enumerable.first = function () {
    return list.first();
  };
  enumerable.last = function () {
    return list.last();
  };
  enumerable.item = function (idx) {
    return list.item(idx);
  };
};



var Allocable = (function (Array) {

  var
    array_from  = ((typeof Array.from == "function") && Array.from) || (function (array_prototype_slice) {
      return function (listType) {

        return array_prototype_slice.call(listType);
      };
    }(Array.prototype.slice))
  ;

  return function (list) { // proxified / bicontextual [Allocable] Mixin.
    var
      allocable = this
    ;
    allocable.valueOf = allocable.toArray = function () {

      return array_from(list);
    };
    allocable.toString = function () {

      return ("" + list);
    };
    allocable.size = function () {

      return list.length;
    };
    Enumerable_first_last_item_proxified.call(allocable, list);
  };

}(Array));



var Queue = (function () {          // [Queue] Module.

  var
    onEnqueue = function (queue, type) {
    //queue.dispatchEvent({type: "enqueue", item: type});
    },
    onDequeue = function (queue, type) {
    //queue.dispatchEvent({type: "dequeue", item: type});
    }/*,
    onEmpty = function (queue) {
    //queue.dispatchEvent({type: "empty"});
    }*/,
    onEmpty = function (queue) {
    //queue.dispatchEvent("empty");
    },

    Queue = function () {           // [Queue] Constructor.
      var
        queue = this,
        list = []
      ;
      queue.enqueue = function (type) {

        list.push(type);
        onEnqueue(queue, type);

        return type;
      };
      queue.dequeue = function () {

        var type = list.shift();
        onDequeue(queue, type);

        (list.length || onEmpty(queue));

        return type;
      };
    //Observable.call(queue);       // applying the [Observable] Mixin.
      Allocable.call(queue, list);  // applying the bicontextual [Allocable] Mixin.
    },

    isQueue = function (type) {
      return !!(type && (type instanceof Queue));
    },
    createQueue = function () {     // [Queue] Factory.
      return (new Queue);
    }
  ;

  return {                          // [Queue] Module.
    isQueue : isQueue,
    create  : createQueue
  };

}());



var q = Queue.create();

//q.addEventListener("enqueue", function (evt) {/* ... */});
//q.addEventListener("dequeue", function (evt) {/* ... */});
//q.addEventListener("empty", function (evt) {/* ... */});


console.log("q : ", q);                     // { .., .., .., }
console.log("q.size() : ", q.size());       // 0
console.log("q.valueOf() : ", q.valueOf()); // []

"the quick brown fox jumped over the lazy dog".split(/\s+/).forEach(function (elm/*, idx, arr*/) {
  console.log("q.enqueue(\"" + elm + "\")", q.enqueue(elm));
});

console.log("q.size() : ", q.size());       // 9
console.log("q.toArray() : ", q.toArray()); // [ .., .., .., ]

console.log("q.first() : ", q.first());     // "the"
console.log("q.last() : ", q.last());       // "dog"
console.log("q.item(2) : ", q.item(2));     // "brown"
console.log("q.item(5) : ", q.item(5));     // "over"

console.log("q.dequeue()", q.dequeue());    // "the"
console.log("q.dequeue()", q.dequeue());    // "quick"
console.log("q.dequeue()", q.dequeue());    // "brown"
console.log("q.dequeue()", q.dequeue());    // "fox"
console.log("q.dequeue()", q.dequeue());    // "jumped"

console.log("q.size() : ", q.size());       // 4
console.log("q.toArray() : ", q.toArray()); // [ .., .., .., ]

console.log("q.first() : ", q.first());     // "over"
console.log("q.last() : ", q.last());       // "dog"
console.log("q.item(2) : ", q.item(2));     // "lazy"
console.log("q.item(5) : ", q.item(5));     // undefined

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