CODEWITHSUNDEEP
javascriptcoffeescript
Ciel
Ciel

Reputation: 17752

How to better understand Coffeescript/JavaScript mixins?

I have been reading up on Mixins using Coffeescript or just plain Javascript from the following sources:

http://arcturo.github.com/library/coffeescript/03_classes.html (near the bottom)

and

http://javascriptweblog.wordpress.com/2011/05/31/a-fresh-look-at-javascript-mixins/

And while I am able to compile the various examples, I have a major question that seems to be preventing me from making progress in comprehending them.

I have no idea what in the world is going on. To start, I will explain the Coffeescript that is confusing me.

moduleKeywords = ['extended', 'included']

    class Module
      @extend: (obj) ->
        for key, value of obj when key not in moduleKeywords
          @[key] = value

        obj.extended?.apply(@)
        this

      @include: (obj) ->
        for key, value of obj when key not in moduleKeywords
          # Assign properties to the prototype
          @::[key] = value

        obj.included?.apply(@)
        this

A number of questions come up here.

  1. First of all, what are we accomplishing with the moduleKeywords variable? I'm not understanding what that is doing.

  2. Secondly, how does extended?.apply(@) work? What is really going on here? I can look at the JavaScript compilation and see the following code ..

Module.extend = function(obj) {
      var key, value, _ref;
      for (key in obj) {
        value = obj[key];
        if (__indexOf.call(moduleKeywords, key) < 0) {
          this[key] = value;
        }
      }
      if ((_ref = obj.extended) != null) {
        _ref.apply(this);
      }
      return this;
    };

Can anyone shed some general light on this?

From deeper down in The Little Book on Coffeescript, I see an implementation. 

ORM = 
  find: (id) ->
  create: (attrs) ->
  extended: ->
    @include
      save: -> 

class User extends Module
  @extend ORM

Here is how I read this:

This is where I get the most lost.

The literal ORM has a method, extended, and then Module is implemented/extended by the 'class' User. So I take this to mean that User has the same shape as Module. That part makes sense so far, simplistic inheritance. But then I get lost on @extend ORM.

@extend is a method on Module, but what is the extended method doing? When is it called? How is it implemented?

Upvotes: 14

Views: 2081

Answers (2)

Bizniztime
Bizniztime

Reputation: 1036

You are confused by meaning and use for extended and included Module keywords. But it is explained in book that those are used as callbacks after extending and including.

So in final example ORM has "extended" callback. The "extend" function will on end call "extended" and pass it @ (or this or in our example User) so @(this.)include will also run on User and it will include function "save".

You could also do the reverse:

ORM = 
  save ->
  included: ->
    @extend
      find: (id) ->
      create: (attrs) ->

class User extends Module
  @include ORM

Upvotes: 0

clyfe
clyfe

Reputation: 23770

  • extend copies the methods from the "module" object onto the object being extended
  • include copies the methods from the "module" object onto the prototype of the object being extended

1 The moduleKeywords is used to protect some methods of the module, so the are not copied to object, because they have special meaning

2 The extended?.apply(@) says that if the module has a property named extended than assume it's a function and then call this function having the "this" in the function equal to @, @ being the extended object, you can think of it as saying something like (although not quite, but it's just an intuition) @.extended() (@ == this in coffeescript)

"apply" function in JS
the existential operator in CS

Upvotes: 5

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