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Prototypical mixin layer


Mixin is an easy way to repeatedly mix functionality into a prototypical JavaScript class. It automatically takes care of the combination of overriding prototype methods and invoking constructors. Moreover, it will notify a mixed in constructor that is has been mixed into another class. This can be used to construct dependent mixin hierarchies.


Using npm:

npm install mixin


var mixin = require("mixin");

function Foo() {

Foo.prototype = {
   t1: function() { return 't1'; }

Foo = mixin(Foo, EventEmitter);

this is the equivalent of:

function Foo() {;

Foo.prototype = Object.create(EventEmitter.prototype);

except that I can define a full set of prototypes in the Foo.prototype statement before I invoke the mixin.


mixin(base, mixed)

The call mixin(base, mixed) returns a new constructor that adds the prototype for mixed at the back of the prototype chain for base and invokes the constructors for both base and mixed in reverse order. If mixed has function property included, then this function will be invoked with this = mixed and the new constructor class as the single argument.

A constructor to be mixed in cannot have a prototype chain of its own (i.e. it can't itself be the product of a mixin), however this functionality can be achieved by calling mixin again inside the included callback. Moreover, the same constructor can not be mixed in twice to the same prototypical inheritance chain.


If ctor is mixed in to another constructor (e.g. by invoking mixin(Base, ctor)) then ctor.included(base) is called with the this set to the mixed in constructor (e.g. ctor) and is passed a single argument, the new constructor created by mixin(). This is extremely useful for creating dependent chains of mixins (i.e. Mixin1 requires Mixin2) since mixin itself can be invoked from within the included call.

If included returns a value, then it is used as the constructor for the call to mixin that invoked this function. This behavior leads to the useful idiom:

Foo.included = function(ctor) {
    return mixin(ctor, EventEmitter);

to mix the functionality of EventEmitter into any constructor that mixes in Foo.

mixin.alias(obj, name, suffix, fun)

Can be used to override a method already defined on obj. It assigns fun to obj[name+'_with_"+suffix] and reassigns the current value of obj[name] to obj[name+'_without_'+suffix]. Finally it sets obj[name] to fun. This is similar to Rails' 'alias_method_chain`.