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Adding properties to DOM objects and functions, and to the window object?

I've read a number of questions that state it is unwise to add properties to DOM element objects; and they seem to make a great deal of sense. The best suggestion I came across to accomplish the same end was to use a Weak Map with the DOM node reference as the key.
That led me to wonder about adding properties to function objects. Is that "safe" to do or should another object or map be used for this also. I'm referring to functions on the window object. Can the property names I add clash with names in the function object in the future?
That brings up a related matter I've been trying to understand which is that some claim that the window object is so cluttered up that it ought not to be added to and scripts should be modules now. Modules appears to be more isolated than my limited experience with simple namespaces. Is there anything to be gained through using scripts of type module over just declaring another object and making your functions methods of that object? That object would still be added to the window object.
Thank you.

That led me to wonder about adding properties to function objects. Is that "safe" to do or should another object or map be used for this also. I'm referring to functions on the window object. Can the property names I add clash with names in the function object in the future?
It's not safe in that, if you do it, others could do it too. For example:
// Your code
RegExp.description = 'some description for the constructor!';
setTimeout(() => {
console.log(RegExp.description);
});
// Someone else's code
RegExp.description = "someone else's description";
The only consolation is that it's pretty unlikely for such a thing to happen.
Similarly, it's pretty unlikely for some random English word that one library creates for itself on the window collides with another word that another library creates for itself on the window - but it's not impossible. For that reason
That object would still be added to the window object.
can be seen as slightly less than optimal on huge pages, where collisions are a real possibility.
Is there anything to be gained through using scripts of type module over just declaring another object and making your functions methods of that object?
Yes, see above. If all of your code is in module scripts, there should be no need to put any object onto the window, in most cases. In addition to the above, module scripts often make projects easier to manage because they make all dependencies explicit through the use of import statements. They're also very easily integrated into a build process like Webpack - and build processes allow for all sorts of neat things that aren't easily done otherwise.

Adding methods to a Prototype vs existing Prototype Javascript [duplicate]

In JavaScript, we have two ways of making a "class" and giving it public functions.
Method 1:
function MyClass() {
var privateInstanceVariable = 'foo';
this.myFunc = function() { alert(privateInstanceVariable ); }
}
Method 2:
function MyClass() { }
MyClass.prototype.myFunc = function() {
alert("I can't use private instance variables. :(");
}
I've read numerous times people saying that using Method 2 is more efficient as all instances share the same copy of the function rather than each getting their own. Defining functions via the prototype has a huge disadvantage though - it makes it impossible to have private instance variables.
Even though, in theory, using Method 1 gives each instance of an object its own copy of the function (and thus uses way more memory, not to mention the time required for allocations) - is that what actually happens in practice? It seems like an optimization web browsers could easily make is to recognize this extremely common pattern, and actually have all instances of the object reference the same copy of functions defined via these "constructor functions". Then it could only give an instance its own copy of the function if it is explicitly changed later on.
Any insight - or, even better, real world experience - about performance differences between the two, would be extremely helpful.

See http://jsperf.com/prototype-vs-this
Declaring your methods via the prototype is faster, but whether or not this is relevant is debatable.
If you have a performance bottleneck in your app it is unlikely to be this, unless you happen to be instantiating 10000+ objects on every step of some arbitrary animation, for example.
If performance is a serious concern, and you'd like to micro-optimise, then I would suggest declaring via prototype. Otherwise, just use the pattern that makes most sense to you.
I'll add that, in JavaScript, there is a convention of prefixing properties that are intended to be seen as private with an underscore (e.g. _process()). Most developers will understand and avoid these properties, unless they're willing to forgo the social contract, but in that case you might as well not cater to them. What I mean to say is that: you probably don't really need true private variables...

In the new version of Chrome, this.method is about 20% faster than prototype.method, but creating new object is still slower.
If you can reuse the object instead of always creating an new one, this can be 50% - 90% faster than creating new objects. Plus the benefit of no garbage collection, which is huge:
http://jsperf.com/prototype-vs-this/59

It only makes a difference when you're creating lots of instances. Otherwise, the performance of calling the member function is exactly the same in both cases.
I've created a test case on jsperf to demonstrate this:
http://jsperf.com/prototype-vs-this/10

You might not have considered this, but putting the method directly on the object is actually better in one way:
Method invocations are very slightly faster (jsperf) since the prototype chain does not have to be consulted to resolve the method.
However, the speed difference is almost negligible. On top of that, putting a method on a prototype is better in two more impactful ways:
Faster to create instances (jsperf)
Uses less memory
Like James said, this difference can be important if you are instantiating thousands of instances of a class.
That said, I can certainly imagine a JavaScript engine that recognizes that the function you are attaching to each object does not change across instances and thus only keeps one copy of the function in memory, with all instance methods pointing to the shared function. In fact, it seems that Firefox is doing some special optimization like this but Chrome is not.
ASIDE:
You are right that it is impossible to access private instance variables from inside methods on prototypes. So I guess the question you must ask yourself is do you value being able to make instance variables truly private over utilizing inheritance and prototyping? I personally think that making variables truly private is not that important and would just use the underscore prefix (e.g., "this._myVar") to signify that although the variable is public, it should be considered to be private. That said, in ES6, there is apparently a way to have the both of both worlds!

You may use this approach and it will allow you to use prototype and access instance variables.
var Person = (function () {
function Person(age, name) {
this.age = age;
this.name = name;
}
Person.prototype.showDetails = function () {
alert('Age: ' + this.age + ' Name: ' + this.name);
};
return Person; // This is not referencing `var Person` but the Person function
}()); // See Note1 below
Note1:
The parenthesis will call the function (self invoking function) and assign the result to the var Person.
Usage
var p1 = new Person(40, 'George');
var p2 = new Person(55, 'Jerry');
p1.showDetails();
p2.showDetails();

In short, use method 2 for creating properties/methods that all instances will share. Those will be "global" and any change to it will be reflected across all instances. Use method 1 for creating instance specific properties/methods.
I wish I had a better reference but for now take a look at this. You can see how I used both methods in the same project for different purposes.
Hope this helps. :)

This answer should be considered an expansion of the rest of the answers filling in missing points. Both personal experience and benchmarks are incorporated.
As far as my experience goes, I use constructors to literally construct my objects religiously, whether methods are private or not. The main reason being that when I started that was the easiest immediate approach to me so it's not a special preference. It might have been as simple as that I like visible encapsulation and prototypes are a bit disembodied. My private methods will be assigned as variables in the scope as well. Although this is my habit and keeps things nicely self contained, it's not always the best habit and I do sometimes hit walls. Apart from wacky scenarios with highly dynamic self assembling according to configuration objects and code layout it tends to be the weaker approach in my opinion particularly if performance is a concern. Knowing that the internals are private is useful but you can achieve that via other means with the right discipline. Unless performance is a serious consideration, use whatever works best otherwise for the task at hand.
Using prototype inheritance and a convention to mark items as private does make debugging easier as you can then traverse the object graph easily from the console or debugger. On the other hand, such a convention makes obfuscation somewhat harder and makes it easier for others to bolt on their own scripts onto your site. This is one of the reasons the private scope approach gained popularity. It's not true security but instead adds resistance. Unfortunately a lot of people do still think it's a genuinely way to program secure JavaScript. Since debuggers have gotten really good, code obfuscation takes its place. If you're looking for security flaws where too much is on the client, it's a design pattern your might want to look out for.
A convention allows you to have protected properties with little fuss. That can be a blessing and a curse. It does ease some inheritance issues as it is less restrictive. You still do have the risk of collision or increased cognitive load in considering where else a property might be accessed. Self assembling objects let you do some strange things where you can get around a number of inheritance problems but they can be unconventional. My modules tend to have a rich inner structure where things don't get pulled out until the functionality is needed elsewhere (shared) or exposed unless needed externally. The constructor pattern tends to lead to creating self contained sophisticated modules more so than simply piecemeal objects. If you want that then it's fine. Otherwise if you want a more traditional OOP structure and layout then I would probably suggest regulating access by convention. In my usage scenarios complex OOP isn't often justified and modules do the trick.
All of the tests here are minimal. In real world usage it is likely that modules will be more complex making the hit a lot greater than tests here will indicate. It's quite common to have a private variable with multiple methods working on it and each of those methods will add more overhead on initialisation that you wont get with prototype inheritance. In most cases is doesn't matter because only a few instances of such objects float around although cumulatively it might add up.
There is an assumption that prototype methods are slower to call because of prototype lookup. It's not an unfair assumption, I made the same myself until I tested it. In reality it's complex and some tests suggest that aspect is trivial. Between, prototype.m = f, this.m = f and this.m = function... the latter performs significantly better than the first two which perform around the same. If the prototype lookup alone were a significant issue then the last two functions instead would out perform the first significantly. Instead something else strange is going on at least where Canary is concerned. It's possible functions are optimised according to what they are members of. A multitude of performance considerations come into play. You also have differences for parameter access and variable access.
Memory Capacity. It's not well discussed here. An assumption you can make up front that's likely to be true is that prototype inheritance will usually be far more memory efficient and according to my tests it is in general. When you build up your object in your constructor you can assume that each object will probably have its own instance of each function rather than shared, a larger property map for its own personal properties and likely some overhead to keep the constructor scope open as well. Functions that operate on the private scope are extremely and disproportionately demanding of memory. I find that in a lot of scenarios the proportionate difference in memory will be much more significant than the proportionate difference in CPU cycles.
Memory Graph. You also can jam up the engine making GC more expensive. Profilers do tend to show time spent in GC these days. It's not only a problem when it comes to allocating and freeing more. You'll also create a larger object graph to traverse and things like that so the GC consumes more cycles. If you create a million objects and then hardly touch them, depending on the engine it might turn out to have more of an ambient performance impact than you expected. I have proven that this does at least make the gc run for longer when objects are disposed of. That is there tends to be a correlation with memory used and the time it takes to GC. However there are cases where the time is the same regardless of the memory. This indicates that the graph makeup (layers of indirection, item count, etc) has more impact. That's not something that is always easy to predict.
Not many people use chained prototypes extensively, myself included I have to admit. Prototype chains can be expensive in theory. Someone will but I've not measured the cost. If you instead build your objects entirely in the constructor and then have a chain of inheritance as each constructor calls a parent constructor upon itself, in theory method access should be much faster. On the other hand you can accomplish the equivalent if it matters (such as flatten the prototypes down the ancestor chain) and you don't mind breaking things like hasOwnProperty, perhaps instanceof, etc if you really need it. In either case things start to get complex once you down this road when it comes to performance hacks. You'll probably end up doing things you shouldn't be doing.
Many people don't directly use either approach you've presented. Instead they make their own things using anonymous objects allowing method sharing any which way (mixins for example). There are a number of frameworks as well that implement their own strategies for organising modules and objects. These are heavily convention based custom approaches. For most people and for you your first challenge should be organisation rather than performance. This is often complicated in that Javascript gives many ways of achieving things versus languages or platforms with more explicit OOP/namespace/module support. When it comes to performance I would say instead to avoid major pitfalls first and foremost.
There's a new Symbol type that's supposed to work for private variables and methods. There are a number of ways to use this and it raises a host of questions related to performance and access. In my tests the performance of Symbols wasn't great compared to everything else but I never tested them thoroughly.
Disclaimers:
There are lots of discussions about performance and there isn't always a permanently correct answer for this as usage scenarios and engines change. Always profile but also always measure in more than one way as profiles aren't always accurate or reliable. Avoid significant effort into optimisation unless there's definitely a demonstrable problem.
It's probably better instead to include performance checks for sensitive areas in automated testing and to run when browsers update.
Remember sometimes battery life matters as well as perceptible performance. The slowest solution might turn out faster after running an optimising compiler on it (IE, a compiler might have a better idea of when restricted scope variables are accessed than properties marked as private by convention). Consider backend such as node.js. This can require better latency and throughput than you would often find on the browser. Most people wont need to worry about these things with something like validation for a registration form but the number of diverse scenarios where such things might matter is growing.
You have to be careful with memory allocation tracking tools in to persist the result. In some cases where I didn't return and persist the data it was optimised out entirely or the sample rate was not sufficient between instantiated/unreferenced, leaving me scratching my head as to how an array initialised and filled to a million registered as 3.4KiB in the allocation profile.
In the real world in most cases the only way to really optimise an application is to write it in the first place so you can measure it. There are dozens to hundreds of factors that can come into play if not thousands in any given scenario. Engines also do things that can lead to asymmetric or non-linear performance characteristics. If you define functions in a constructor, they might be arrow functions or traditional, each behaves differently in certain situations and I have no idea about the other function types. Classes also don't behave the same in terms as performance for prototyped constructors that should be equivalent. You need to be really careful with benchmarks as well. Prototyped classes can have deferred initialisation in various ways, especially if your prototyped your properties as well (advice, don't). This means that you can understate initialisation cost and overstate access/property mutation cost. I have also seen indications of progressive optimisation. In these cases I have filled a large array with instances of objects that are identical and as the number of instances increase the objects appear to be incrementally optimised for memory up to a point where the remainder is the same. It is also possible that those optimisations can also impact CPU performance significantly. These things are heavily dependent not merely on the code you write but what happens in runtime such as number of objects, variance between objects, etc.

What's wrong with this style of coding JavaScript? (closures vs. prototypes)

We have been debating how best to handle objects in our JS app, studying Stoyan Stefanov's book, reading endless SO posts on 'new', 'this', 'prototype', closures etc. (The fact that there are so many, and they have so many competing theories, suggests there is no completely obvious answer).
So let's assume the we don't care about private data. We are content to trust users and developers not to mess around in objects outside the ways we define.
Given this, what (other than it seeming to defy decades of OO style and history) would be wrong with this technique?
// namespace to isolate all PERSON's logic
var PERSON = {};
// return an object which should only ever contain data.
// The Catch: it's 100% public
PERSON.constructor = function (name) {
return {
name: name
}
}
// methods that operate on a Person
// the thing we're operating on gets passed in
PERSON.sayHello = function (person) {
alert (person.name);
}
var p = PERSON.constructor ("Fred");
var q = PERSON.constructor ("Me");
// normally this coded like 'p.sayHello()'
PERSON.sayHello(p);
PERSON.sayHello(q);
Obviously:
There would be nothing to stop someone from mutating 'p' in unholy
ways, or simply the logic of PERSON ending up spread all over the place. (That is true with the canonical 'new' technique as well).
It would be a minor hassle to pass 'p' in to every function that you
wanted to use it.
This is a weird approach.
But are those good enough reasons to dismiss it? On the positive side:
It is efficient, as (arguably) opposed to closures with repetitive function declaration.
It seems very simple and understandable, as opposed to fiddling with
'this' everywhere.
The key point is the foregoing of privacy. I know I will get slammed for this, but, looking for any feedback. Cheers.

There's nothing inherently wrong with it. But it does forgo many advantages inherent in using Javascript's prototype system.
Your object does not know anything about itself other than that it is an object literal. So instanceof will not help you to identify its origin. You'll be stuck using only duck typing.
Your methods are essentially namespaced static functions, where you have to repeat yourself by passing in the object as the first argument. By having a prototyped object, you can take advantage of dynamic dispatch, so that p.sayHello() can do different things for PERSON or ANIMAL depending on the type Javascript knows about. This is a form of polymorphism. Your approach requires you to name (and possibly make a mistake about) the type each time you call a method.
You don't actually need a constructor function, since functions are already objects. Your PERSON variable may as well be the constructor function.
What you've done here is create a module pattern (like a namespace).
Here is another pattern that keeps what you have but supplies the above advantages:
function Person(name)
{
var p = Object.create(Person.prototype);
p.name = name; // or other means of initialization, use of overloaded arguments, etc.
return p;
}
Person.prototype.sayHello = function () { alert (this.name); }
var p = Person("Fred"); // you can omit "new"
var q = Person("Me");
p.sayHello();
q.sayHello();
console.log(p instanceof Person); // true
var people = ["Bob", "Will", "Mary", "Alandra"].map(Person);
// people contains array of Person objects

Yeah, I'm not really understanding why you're trying to dodge the constructor approach or why they even felt a need to layer syntactical sugar over function constructors (Object.create and soon classes) when constructors by themselves are an elegant, flexible, and perfectly reasonable approach to OOP no matter how many lame reasons are given by people like Crockford for not liking them (because people forget to use the new keyword - seriously?). JS is heavily function-driven and its OOP mechanics are no different. It's better to embrace this than hide from it, IMO.
First of all, your points listed under "Obviously"
Hardly even worth mentioning in JavaScript. High degrees of mutability is by-design. We're not afraid of ourselves or other developers in JavaScript. The private vs. public paradigm isn't useful because it protects us from stupidity but rather because it makes it easier to understand the intention behind the other dev's code.
The effort in invoking isn't the problem. The hassle comes later when it's unclear why you've done what you've done there. I don't really see what you're trying to achieve that the core language approaches don't do better for you.
This is JavaScript. It's been weird to all but JS devs for years now. Don't sweat that if you find a better way to do something that works better at solving a problem in a given domain than a more typical solution might. Just make sure you understand the point of the more typical approach before trying to replace it as so many have when coming to JS from other language paradigms. It's easy to do trivial stuff with JS but once you're at the point where you want to get more OOP-driven learn everything you can about how the core language stuff works so you can apply a bit more skepticism to popular opinions out there spread by people who make a side-living making JavaScript out to be scarier and more riddled with deadly booby traps than it really is.
Now your points under "positive side,"
First of all, repetitive function definition was really only something to worry about in heavy looping scenario. If you were regularly producing objects in large enough quantity fast enough for the non-prototyped public method definitions to be a perf problem, you'd probably be running into memory usage issues with non-trivial objects in short order regardless. I speak in the past tense, however, because it's no longer really a relevant issue either way. In modern browsers, functions defined inside other functions are actually typically performance enhancing due to the way modern JIT compilers work. Regardless of what browsers you support, a few funcs defined per object is a non-issue unless you're expecting tens of thousands of objects.
On the question of simple and understandable, it's not to me because I don't see what win you've garnered here. Now instead of having one object to use, I have to use both the object and it's pseudo-constructor together which if I weren't looking at the definition would imply to me the function that you use with a 'new' keyword to build objects. If I were new to your codebase I'd be wasting a lot of time trying to figure out why you did it this way to avoid breaking some other concern I didn't understand.
My questions would be:
Why not just add all the methods in the object literal in the constructor in the first place? There's no performance issue there and there never really has been so the only other possible win is that you want to be able to add new methods to person after you've created new objects with it, but that's what we use prototype for on proper constructors (prototype methods btw are great for memory in older browsers because they are only defined once).
And if you have to keep passing the object in for the methods to know what the properties are, why do you even want objects? Why not just functions that expect simple data structure-type objects with certain properties? It's not really OOP anymore.
But my main point of criticism
You're missing the main point of OOP which is something JavaScript does a better job of not hiding from people than most languages. Consider the following:
function Person(name){
//var name = name; //<--this might be more clear but it would be redundant
this.identifySelf = function(){ alert(name); }
}
var bob = new Person();
bob.identifySelf();
Now, change the name bob identifies with, without overwriting the object or the method, which are both things you'd only do if it were clear you didn't want to work with the object as originally designed and constructed. You of course can't. That makes it crystal clear to anybody who sees this definition that the name is effectively a constant in this case. In a more complex constructor it would establish that the only thing allowed to alter or modify name is the instance itself unless the user added a non-validating setter method which would be silly because that would basically (looking at you Java Enterprise Beans) MURDER THE CENTRAL PURPOSE OF OOP.
Clear Division of Responsibility is the Key
Forget the key words they put in every book for a second and think about what the whole point is. Before OOP, everything was just a pile of functions and data structures all those functions acted on. With OOP you mostly have a set of methods bundled with a set of data that only the object itself actually ever changes.
So let's say something's gone wrong with output:
In our strictly procedural pile of functions there's no real limit to the number of hands that could have messed up that data. We might have good error-handling but one function could branch in such a way that the original culprit is hard to track down.
In a proper OOP design where data is typically behind an object gatekeeper I know that only one object can actually make the changes responsible.
Objects exposing all of their data most of the time is really only marginally better than the old procedural approach. All that really does is give you a name to categorize loosely related methods with.
Much Ado About 'this'
I've never understood the undue attention assigned to the 'this' keyword being messy and confusing. It's really not that big of a deal. 'this' identifies the instance you're working with. That's it. If the method isn't called as a property it's not going to know what instance to look for so it defaults to the global object. That was dumb (undefined would have been better), but it not working properly in that scenario should be expected in a language where functions are also portable like data and can be attached to other objects very easily. Use 'this' in a function when:
It's defined and called as a property of an instance.
It's passed as an event handler (which will call it as a member of the thing being listened to).
You're using call or apply methods to call it as a property of some other object temporarily without assigning it as such.
But remember, it's the calling that really matters. Assigning a public method to some var and calling from that var will do the global thing or throw an error in strict mode. Without being referenced as object properties, functions only really care about the scope they were defined in (their closures) and what args you pass them.

Efficiency of Plain Functions vs. Immediate Functions?

Someone mentioned that immediate or self-executing functions have to store the whole stack. Is this true...If so what are the pros and cons of using something like the module pattern (which is based on an immediate function) vs. a plain function?
A function is inherently private, but you can return items that you want to be public, so it can handle privacy.
The main difference I see, is that you don't have global imports or the ability to make sure that the developer ( wait that's me ) uses new with the function ( or it is complicated ).
In general when trying to provide privacy and state when should one use the module pattern and when should one just use a plain function?
A second side question is does a function provide state when used with new?

Any function closure that persists because there are lasting references to variables or functions inside it occupies some amount of memory. In today's computers (even phones), this amount of memory is generally insignificant unless you're somehow repeating it thousands of times. So, using the language features to solve your design problems is generally more important than worrying about this amount of memory.
When you say "the whole stack", the calling stack for a top-level self-executing function is very small. There's really nothing else on the stack except for the one function that's being called.
A function is also an object. So, when it's used with new, it creates a new object that can have state (it's properties) if you assign values to those properties. That's one of the main ways that objects are created in javascript. You can either call a function and examine it's return value or you can use it with new and the function serves as the constructor for a new object. A given function is usually designed to be used in one way or the other, not both.
The module pattern is generally used to control which variables are public and when making them public to put them into a structured namespace that uses very few top-level global variables. It isn't really something you choose instead of self-executing functions because they don't really solve the same problem. You can read more about the module pattern here: http://www.yuiblog.com/blog/2007/06/12/module-pattern/
You can read about a number of the options here: http://www.adequatelygood.com/2010/3/JavaScript-Module-Pattern-In-Depth and http://www.klauskomenda.com/code/javascript-programming-patterns/.
It is easier to discuss the pros/cons of a given technique in light of a specific problem that one is trying to solve or a specific design issue rather than a generic discussion of which is better when the things you've asked about are not really solving equivalent issues.
The best reference I know of for protected and private members (which can be hacked into javascript, but are not a core language feature) is this one: http://javascript.crockford.com/private.html. You are making tradeoffs when you use this method instead of the default prototype feature of the language, but you can achieve privacy if you really need it. But, you should know that javascript was not build with private or protected methods in mind so to get that level of privacy, you're using some conventions about how you write your code to get that.

Why were namespaces removed from ECMAScript consideration?

Namespaces were once a consideration for ECMAScript (the old ECMAScript 4) but were taken out. As Brendan Eich says in this message:
One of the use-cases for namespaces in
ES4 was early binding (use namespace
intrinsic), both for performance and
for programmer comprehension -- no
chance of runtime name binding
disagreeing with any earlier
binding. But early binding in any
dynamic code loading scenario like
the web requires a prioritization or
reservation mechanism to avoid early
versus late binding conflicts.
Plus, as some JS implementors have
noted with concern, multiple open
namespaces impose runtime cost unless
an implementation works
significantly harder.
For these reasons, namespaces and
early binding (like packages before
them, this past April) must go.
But I'm not sure I understand all of that. What exactly is a prioritization or reservation mechanism and why would either of those be needed? Also, must early binding and namespaces go hand-in-hand? For some reason I can't wrap my head around the issues involved. Can anyone attempt a more fleshed out explanation?
Also, why would namespaces impose runtime costs? In my mind I can't help but see little difference in concept between a namespace and a function using closures. For instance, Yahoo and Google both have YAHOO and google objects that "act like" namespaces in that they contain all of their public and private variables, functions, and objects within a single access point. So why, then, would a namespace be so significantly different in implementation? Maybe I just have a misconception as to what a namespace is exactly.
For the sake of the bounty I'd like to know two things:
Does a namespace need early binding?
How does a namespace implementation
differ from an object with
private members?

If you declare a variable within a closure after a definition of a function will call that variable, it still uses the scoped variable.
function ShowMe() {
alert(myVar); //alerts "cool"
}
var myVar = "cool";
This process would get another layer of complicated with regards to namespacing.
Beyond this there are numerous namespace methods along with expand/applyIf etc that can do a lot of the same functionality. namespace() in ExtJS, or $.extend in jQuery for example. So, it may be a nice to have, but isn't an absolute need within the constructs of the language. I think that formalizing some of the extensions for Array, and support for ISO-8601 dates in Date are far more important myself. Over having to simply check each namespace layer for definition...
window.localization = $.extend(window.localization || {}, {
...
});
window.localization.en = $.extend(window.localization.en || {}, {
...
});

OK first Some of the terminology:
Early binding -- check/validate when the line of code is parsed.
Late binding -- check/validate when the line of code is executed.
Prioritisation/reservation --I think they mean if you do early binding and check the name spaces as the code is parsed there must be a process to validate a variable name when the variable is added dynamically later on and a different set of rules for what is valid as there may be several scopes involved at this point.
I am frankly surprised that only these quite technical reasons were given. Take the common case:-
onclick=' var mymark = "donethat";'
What namespace should "mymark" belong to? There doesnt seem to be any easy answer to that question. Especially given that the code snippet:-
window.forms.myform.mybutton.onClick = ' var mymark = "donethat";'
Should put the variable in the same name space.