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Multiple inheritance in Javascript with prototype chain

I have a JavaScript MVC design, implemented using prototypes, where different items may be displayed in different ways as decided by the controller. For example an 'Event' item may be displayed with the views ViewTabs or ViewSimple. Class hierarchy:
ViewBase
- ViewTabs
-- EventViewTabs, which implements EventViewTabs.Validate
- ViewSimple
-- EventViewSimple, which implements EventViewSimple.Validate
Deciding whether to use EventViewTabs or EventViewSimple is done by a EventController. My problem is: I have a Validate method for checking inputs from the Event views, but this method is identical for the EventViewTabs and the EventViewSimple views. Where should I put Validate in order to avoid duplication? I cannot put it in ViewBase, as other items (e.g. User) also inherit from this class.
Seems I need multiple inheritance for this, but is there a smarter way to do it? I have a feeling I'm overlooking something obvious.

You're missing composition. Inheritance isn't the answer to all issues about code reuse to avoid copy-paste programming.
Let's say you've a View base prototype:
function View() { }
If you want this view to support validation, you can inject the validation dependency in the constructor function:
function View(validator) {
this.validator = validator;
}
View.prototype = {}; // A lot of functions here
That is, now any view which inherits View's prototype will have an associated validator. In other words: you don't need to derive two prototypes in your concrete view (you don't need and you can't do it anyway).
In the other hand, in terms of object-oriented programming, it wouldn't make sense to derive from Validator to create a View.
When you say a view has a validator, since you're using has as verb, you're talking about an association (a form of composition). Alternatively, when you say my main screen is as view, we're talking about an inheritance, because a specific view must be also a view, so it needs base view's members to act like a view.

Basically your validator could be tailor-made with the type it has to work with. In UML, it's called composition. I figure out your code as follows:
function Validator {}
Validator.prototype.validate = function(arg) {
//arg is no longer inputs
return true|false; //the ultimate output along with additional information;
}
function EventViewTabsValidator() {}
EventViewTabsValidator.prototype = Object.extend(Validator.prototype); //inheritance
EventViewTabsValidator.prototype.constructor = EventViewTabsValidator; //enforce the constructor to point to your derived type
EventViewTabsValidator.prototype.validate = function() {
var inputs = $('inputs');
var param = 'do some stuff specific to EventViewTabsValidator based on the inputs';
return Validator.prototype.validate.call(this, param); //pass param, not inputs
}
function EventViewSimpleValidator() {}
EventViewSimpleValidator.prototype = Object.extend(Validator.prototype); //inheritance
EventViewSimpleValidator.prototype.constructor = EventViewSimpleValdiator; //enforce the constructor to point to your derived type
EventViewSimpleValidator.prototype.validate = function() {
var inputs = $('inputs');
var param = 'do some stuff specific to EventViewSimpleValidator based on the inputs';
return Validator.prototype.validate.call(this, param); //pass param, not inputs
}
function EventViewTabs() {
this.validator = null; //see init
}
EventViewTabs.prototype.init = function() {
this.validator = new EventViewTabsValidator();
}
function EventViewSimple() {
this.validator = null; //see init
}
EventViewSimple = function() {
this.validator = new EventViewSimpleValidator();
}
Your could abstract up both types to a base EventView, which could expose this.validator.
Your instance of EventController will call:
var simple = new EventViewSimple();
simple.validator.validate();
var tabs = new EventViewTabs();
tabs.validator.validate();
Whatever the EventView instance, they implement their own specific validator that can be called in a generic way.

One approach is to use mixins to add the other behavior (this is the ruby approach, and is also used by react.js and react.rb) You can google for javascript+mixins and find some excellent tutorials like this one: http://raganwald.com/2014/04/10/mixins-forwarding-delegation.html
For your specific case validate (or perhaps validator) would be the mixin.

Why not to do something like this:
ViewBase
ViewBase
-EventValidator, which implements Validate
--ViewTabs
---EventViewTabs
--ViewSimple
---EventViewSimple.
Also consider to use composition over inheritance see this video

Circular reference memory leak?

I am in doubt if the following design pattern would cause a memory leak.
I have been using it for some time with success, but I haven't seen this pattern used by others, so I'd like some confirmation if you see something wrong with it.
As from next month I have to start working on a large project, and I want to know for sure that I can use this without problems, or if I should use another strategy.
controller.js:
var Controller = function(options){
};
Controller.prototype.makeView = function(options){
options.controller = this;
options.otheroption = options.otheroption;
var view = new View(options);
};
Controller.prototype.getModel = function(options){
//--- Get model ---
var model = new Model();
var promise = model.fetch();
return promise;
});
view.js:
var View = Backbone.View.extend({
initialize: function(options){
this.controller = options.controller;
this.otheroption = options.otheroption;
},
getModel: function(){
var promise = this.controller.getModel();
promise.done(_.bind(function(model){
//Do something with the returned model instance
}, this));
};
});
Instantiate controller, eg. from the router, or another controller:
//--- Instantiate the controller and build the view ---//
var controller = new Controller();
controller.makeView(options)
To me, this doesn't look like a circular reference, because both the controller and view are declared as a local variable.
Yet the instantiated view can access the controller functions, which allows me to isolate the RESTful server interactions via models / collections that the view uses.
For me it would seem as if the only reference remaining would be the view that keeps a reference to the controller object.
What I do afterwards is clean up the view (I destroy the instance and its references when I don't need it anymore.
Your opinion on this pattern is highly appreciated.
My purpose is to isolate creation of views / server interactions in separate controller files: if you see holes in my method and have a better way of doing it, please share.
Thanks.

Short answer: There is no memory leak problem in the code you have posted. The view holds a reference to the controller, but not vice versa. So as long as the controller lives longer than the view, that reference does not keep your objects from being garbage-collected. I don't see a circular reference anywhere in your code.
Longer answer: The pitfalls would be in the code you haven't posted. In particular, any event handlers in your view must be cleaned up properly, otherwise your views never fade into oblivion. But you have said in your question that you clean up your view, so I guess you are aware of that sort of problem.

What controller doing is here looks like a utility to me. Could have been easily managed by a global level singleton. I see some issues in first glance.
Code repetition, assuming you would creating separate Controller for different types of Models and Views, makeView and getModel code needs to be repeated for each controller. If you extending from a BaseController, then you need to pass View and Model Class to getModel and makeView functions.
How do you handle a use-case where you have to use same model in different Views?
makeView and getModel is designed assuming for each makeView there would be a getModel call, in assumed order
I would rather write a utility function which can create and deploy views for me.
var deployView = function(view, config){
//do the view rendering
view.render();
view.$el.appendTo(config.el);
}
var createView = function(config) {
var view;
var viewType = 'model';
if (config.collection || config.Collection) {
viewType = 'collection';
}
if (viewType === 'model') {
if (config.Model) {
config.model = new config.Model(config.modelAttributes);
//fetch if needed
}
} else {
if (config.Collection) {
config.collection = new config.Collection(config.items);
//fetch if needed
}
}
var filteredConfig = _.omit(config, 'Collection', 'Model', 'View');
view = new config.View(filteredConfig);
deployView(view, filteredConfig)
}

JavaScript implementations haven't had a problem with circular references for a long time. (IE6 did have a memory leak from circular references if I recall correctly, which wasn't shared by any other major browser from that period.)
Modern JavaScript implementations perform garbage collection through a "mark and sweep" algorithm. First they scan through your web app's entire memory structure starting from the global object, and mark everything they find. Then they sweep through every object stored in memory and garbage collect anything that wasn't marked. As long as there isn't a reference to your object from the global object or any stored function, it can be garbage collected.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Memory_Management#Mark-and-sweep_algorithm
You're probably thinking of a reference counting-based implementation, which does have issues with memory leaks from circular references. In that implementation as long as one object contained a reference to another, that second object can't be garbage collected. That method was once used in web browsers but not anymore.
Nowadays, most memory leaks are from globally-accessible objects you forget to clean up and accidentally retaining data in function closures (a function that creates another function and passes/saves it somewhere). Since the closure's local variables can be accessed by the function created inside of them, they have to be retained as long as that function exists.
So go ahead and add all the circular references you want. Unless you need to target IE6, your code's fine.

Backbone and RequireJS conflicts - instances or constructors?

Could someone explain the fundamental difference between:
define(['backbone'], function(Backbone) {
MyModel = Backbone.Model.extend({
});
});
define(['backbone', 'models/mymodel'], function(Backbone){
var app = Backbone.View.extend({
initialize: function() {
var model = new MyModel();
}
});
});
and:
define(['backbone'], function(Backbone) {
var MyModel = Backbone.Model.extend({
});
return MyModel;
});
define(['backbone', 'models/mymodel'], function(Backbone, MyModel){
var app = Backbone.View.extend({
initialize: function() {
var model = new MyModel();
}
});
});
In the former, the first module simply defines MyModel. In the latter, it's created as a variable and returned, and the second module needs to have it put in the parameters when imported.
RequireJS examples I see around seem to vary between the two, but I don't really understand the difference - does one return an instance and the other a constructor?
In my application I didn't even notice that I was actually using both ways in different places, and I think it was causing problems. I was using a lot of
self = this
self.model.doSomething
inside my views and models, and as my app got bigger, I started getting errors because there were conflicts with definitions of self.

Short Version: 1st version == wrong.
Medium Version: The first one bypasses Require entirely by using global variables, while the second one actually uses Require.
Long version:
The way Backbone modules work is that you run "define", pass it a function (and usually an array of dependencies also), and whatever gets returned from that function is defined as that module. So if I do:
// Inside foo.js
define([], function() {
return 1;
});
I've defined the "foo" module to be 1, so if elsewhere I do:
define(['foo'], function(foo) {
alert(foo); // alerts 1
});
Your first version doesn't return anything, so it's not actually creating a Require module at all.
How does it work then? Well, in that version you do:
MyModel = Backbone.Model.extend({
NOT:
var MyModel = Backbone.Model.extend({
So that's really the same as doing:
window.MyModel = Backbone.Model.extend({
Then when the second part of the code runs, it access window.MyModel, and works ... but it's completely bypassing Require.js in the process.
I think the most important thing to takeaway is: ALWAYS DECLARE (ie. var) YOUR JAVASCRIPT VARIABLES. I don't agree with everything Crockford says, but he's dead right on this one. You will get lots of bugs (with Require and without) if you don't make this a habit.
Beyond that, the next most important thing is probably: ALWAYS RETURN SOMETHING FROM THE FUNCTION YOU PASS TO define. There are certain special cases where you don't want to return anything, but unless you are deliberately trying to solve one of those cases you should always return something to define the module.
Finally, if you're using Require, every variable in your code should either:
Come from the define function (ie. it should be an argument variable from the function that you pass to define), or
It should be declared (ie. var-ed ) inside that file
If you use JSLint or 'use strict'; (as Valentin Nemcev suggested), or if you use an editor like Eclipse, your tools can help you ensure this (and in fact make it easy to ensure).

MyModel = Backbone.Model.extend({});
Here you are not returning a constructor, you are defining a global variable and accessing it later in different module.
Actually it is wrong, it works by accident. You should return your modules from define and access them via parameters in other modules.
Like this:
return Backbone.Model.extend({});
You should use strict mode to avoid problems with global variables in JS.
Also, constructor in JS is just a function that is meant to be run with new. Backbone extend always returns a constructor function, and you create a model instance by calling the constructor with new, like you are doing in both examples.

Passing in functions to Backbone.View.extend

Recently I'm having an argument with some co-workers about something that I
find incorrect.
We're using Backbone in a large application and my way to create views is
the 'standard' backbone way :
var MyView = Backbone.View.extend({
className: 'foo',
initialize: function() {
_.bindAll(this, 'render' /* ... more stuff */);
},
render: function() {
/* ... render, usually
using _.template and passing
in this.model.toJSON()... */
return this;
}
});
But someone in the team recently decided to do it this way :
var MyView = Backbone.View.extend( (function() {
/* 'private stuff' */
function bindMethods(view) {
_.bindAll(view, /* ... more stuff */);
};
function render(view) {
/* ... render, usually
using _.template and passing
in view.model.toJSON()... */
};
return {
className: 'foo',
initialize: function() {
bindMethods(this);
render(this);
}
};
}());
That's the idea in pseudo-code .
Having read the BB source and read tutorials, articles I find this to be a
bad practice (for me it makes no sense), but I'd love some feedback from
other Backbone developers/users
Thanks in advance

One benefit I see from using the closure is providing a private scope for variables and functions that you don't want to be accessible from code outside the view.
Even so, I haven't seen many Backbone apps use a closure to define a view/model/collection etc.
Here's an email from Jeremy Ashkenas concerning this issue as well.
Yes, using closures to create instances of objects with private variables is possible in JavaScript. But it's a bad practice, and should be avoided. This has nothing to do with Backbone in particular; it's the nature of OOP in JavaScript.
If you use the closure pattern (also known as the "module" pattern), you're creating a new copy of each function for each instance you create. This completely ignores prototypes, and is terribly inefficient both in terms of speed and especially in terms of memory use. If you make 10,000 models, you'll also have 10,000 copies of each member function. With prototypes (with Backbone.Model.extend), you'll only have a single copy of each member function, even if there are 10,000 instances of the class.

I totally agree with Paul here. Sometimes you may find it necessary to define methods and properties that are private and can't be messed with from outside. I think it depends wether you need this scoping mechanism in your class or not. Mixing both approaches, with respect to the requirements you have for the class wouldn't be so bad, would it?

Dependency Injection with RequireJS

How much can I stretch RequireJS to provide dependency injection for my app? As an example, let's say I have a model that I want to be a singleton. Not a singleton in a self-enforcing getInstance()-type singleton, but a context-enforced singleton (one instance per "context"). I'd like to do something like...
require(['mymodel'], function(mymodel) {
...
}
And have mymodel be an instance of the MyModel class. If I were to do this in multiple modules, I would want mymodel to be the same, shared instance.
I have successfully made this work by making the mymodel module like this:
define(function() {
var MyModel = function() {
this.value = 10;
}
return new MyModel();
});
Is this type of usage expected and common or am I abusing RequireJS? Is there a more appropriate way I can perform dependency injection with RequireJS? Thanks for your help. Still trying to grasp this.

This is not actually dependency injection, but instead service location: your other modules request a "class" by a string "key," and get back an instance of it that the "service locator" (in this case RequireJS) has been wired to provide for them.
Dependency injection would involve returning the MyModel constructor, i.e. return MyModel, then in a central composition root injecting an instance of MyModel into other instances. I've put together a sample of how this works here: https://gist.github.com/1274607 (also quoted below)
This way the composition root determines whether to hand out a single instance of MyModel (i.e. make it singleton scoped) or new ones for each class that requires it (instance scoped), or something in between. That logic belongs neither in the definition of MyModel, nor in the classes that ask for an instance of it.
(Side note: although I haven't used it, wire.js is a full-fledged dependency injection container for JavaScript that looks pretty cool.)
You are not necessarily abusing RequireJS by using it as you do, although what you are doing seems a bit roundabout, i.e. declaring a class than returning a new instance of it. Why not just do the following?
define(function () {
var value = 10;
return {
doStuff: function () {
alert(value);
}
};
});
The analogy you might be missing is that modules are equivalent to "namespaces" in most other languages, albeit namespaces you can attach functions and values to. (So more like Python than Java or C#.) They are not equivalent to classes, although as you have shown you can make a module's exports equal to those of a given class instance.
So you can create singletons by attaching functions and values directly to the module, but this is kind of like creating a singleton by using a static class: it is highly inflexible and generally not best practice. However, most people do treat their modules as "static classes," because properly architecting a system for dependency injection requires a lot of thought from the outset that is not really the norm in JavaScript.
Here's https://gist.github.com/1274607 inline:
// EntryPoint.js
define(function () {
return function EntryPoint(model1, model2) {
// stuff
};
});
// Model1.js
define(function () {
return function Model1() {
// stuff
};
});
// Model2.js
define(function () {
return function Model2(helper) {
// stuff
};
});
// Helper.js
define(function () {
return function Helper() {
// stuff
};
});
// composition root, probably your main module
define(function (require) {
var EntryPoint = require("./EntryPoint");
var Model1 = require("./Model1");
var Model2 = require("./Model2");
var Helper = require("./Helper");
var entryPoint = new EntryPoint(new Model1(), new Model2(new Helper()));
entryPoint.start();
});

If you're serious about DI / IOC, you might be interested in wire.js: https://github.com/cujojs/wire
We use a combination of service relocation (like Domenic describes, but using curl.js instead of RequireJS) and DI (using wire.js). Service relocation comes in very handy when using mock objects in test harnesses. DI seems the best choice for most other use cases.

Not a singleton in a self-enforcing getInstance()-type singleton, but
a context-enforced singleton (one instance per "context").
I would recommend it only for static objects. It's perfectly fine to have a static object as a module that you load using in the require/define blocks. You then create a class with only static properties and functions. You then have the equivalent of the Math Object that has constants like PI, E, SQRT and functions like round(), random(), max(), min(). Great for creating Utility classes that can be injected at any time.
Instead of this:
define(function() {
var MyModel = function() {
this.value = 10;
}
return new MyModel();
});
Which creates an instance, use the pattern for a static object (one where values are always the same as the Object never gets to be instantiated):
define(function() {
return {
value: 10
};
});
or
define(function() {
var CONSTANT = 10;
return {
value: CONSTANT
};
});
If you want to pass an instance (the result of using a Module that have return new MyModel();), then, within an initialize function, pass a variable that capture the current state / context or pass on the Object that contains information on state / context that your modules needs to know about.