Javascript mixins when using the module pattern

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;
})();

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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.