Develop Reference

How do JS `localStorage` property descriptors actually work

Is there any place where it is specified exactly how the properties of localStorage work? The only thing I found in the standards is:
value = storage.getItem (key)
value = storage[key]
Returns the current value associated with the given key, or null if the given key does not exist.
storage.setItem (key, value)
storage[key] = value
Sets the value of the pair identified by key to value, creating a new key/value pair if none > existed for key previously.
But it doesn’t say anything about what happens if key is a special property name like length or setItem (or __proto__ or constructor).
In practice this seems to work as one would expect: “special” properties work as usual, and only “unused” property names can be accessed with the bracket syntax.
But stuff gets a bit weird if you look at the details. At least on Chrome, if you add a value using as key a “special” property name, like localStorage.setItem('length', 'test'), then, somehow, Object.keys(localStorage) does include 'length' in the result. Also, if you try Object.getOwnPropertyNames(localStorage), 'length' is included. But if you try Object.getOwnPropertyDescriptors(localStorage), no property named 'length' is returned.
I can’t find any specification, or even description, of how these work. The ECMA standard mentions that “exotic” objects can override “abstract operations” with special behavior. But I couldn’t find any specs for how that is supposed to work, exactly, with localStorage. All that I could see are cursory mentions about behavior in specific browsers, and those are usually old and I presume obsolete.
So, in short: Is the detailed behavior of the localStorage specified anywhere?

Array.length vs arrayinstance.length in javascript

I'm a bit confused by the Array.length property (i.e. a property named length on the Array function object) and array_instance.length (i.e. a property named length on instance of array object)
So what is difference between the two length property and when should/shouldn't we use them?
Edit 1:
there is also a length property on Array.prototype object. I am so confused.
Edit 2
Just to paint a clearer picture, here are the different length properties I have found
Edit 3
This is one of the follow up questions I asked in the comment section, but I think the question is important to fully understanding length property, so I have copy-pasted here in the main section
Follow up question:
Array.prototype.hasOwnProperty('length') and Array_instance.hasOwnProperty('length') return true, does that mean there are two length properties, one on array_instance, one on Array.prototype object, with the array_instance.length overshadowning the Array.prototype.length?

functions have a .length property which corresponds to how many arguments they are expecting. For example:
const unary = (a) => {
}
const binary = (a, b) => {
}
console.log(unary.length);
console.log(binary.length);
So the Array constructor has a length of 1 because it expects one parameter to be passed to it (namely, the size of the array).
array objects also have a .lengthproperty, which is unrelated other than having the same name. This property says how large the array currently is.

I really think a lot of the other answers have covered everything needed here, but as it seems the OP hasn't had what they see a a clear answer, I will try to set everything out, somewhat extensively - but as clearly as I can - in order to clarify. (Apologies if anyone thinks I am "stealing" their answer - I assure you that this is not the intention, and I'm deliberately not looking at them as I type this, but I've certainly read most of them and even upvoted a few.)
Array.length
This has already been well-covered above. Array is a native JS function, which you can use for creating arrays. It's less common then simply defining an array literal (and as far as I know there is no reason it would ever be preferable), but instead of var a = [1,2,3] you are allowed to do this:
var a = Array(1,2,3);
console.log(a);
Note in passing that you don't want to do this to create a singleton array, there is an utterly mad gotcha of a special case when you supply exactly one parameter which happens to be an integer:
var a = Array(5);
console.log(a);
Although that shows what appears to be an array of 5 undefined values in whatever JS console implementation SO uses, that's not actually quite what has been created (nor what is displayed in the current version of Chrome). I'm getting way off-topic but I'll refer you to Kyle Simpson's excellent walkthrough of the madness.
Anyway, since Array is a function, as others have already observed, it has a length property as all functions do. I'm really not sure why it evaluates to 1 though - for a user-defined function it is the number of arguments the function was formally declared with, but since Array like all native JS functions isn't actually implemented in JS, I couldn't tell you how the implementation actually works. Clearly it can take any number of arguments and thus is a rather "exotic" function. I don't think that Array.length would ever be useful, no matter what value was arbitrarily assigned to it, but it seems that most implementations go for 1, even if the specification leaves it open. (I'm not enough of a spec devotee to know if this is actually defined in there or left up to implementations.)
arrayinstance.length
This is just the feature of arrays that we know and use all the time. All JS arrays get this property - note that, although it is a property rather than a method (that is, it is not a function), it nevertheless "auto-updates" as the array gets longer/shorter:
var a = [1,2,3];
console.log(a.length);
a.push(4);
console.log(a.length);
a.pop();
a.pop();
console.log(a);
console.log(a.length);
Although as I said, Javascript's native constructors are not implemented in terms of JS itself, you could implement this kind of behaviour by defining a getter (at least since ES5).
Array.prototype.length
To fully explain what Array.prototype (and similar objects) is would take me deep into how Javascript's object system work. Suffice to say here that, unlike class-based languages (and JS does not have classes, even in ES6, despite the class syntax allowing us often to pretend it does), JS does not have the usual concept of "inheritance" that so-called OO languages do. In JS's version (sometimes called "prototypal inheritance"), what happens it that each object has an "internal prototype" which references some other object - and if you try to access a property on that object which it doesn't have, the JS engine will look at that "prototype object" for the property and use its value instead.
It's actually a very simple mechanism, but there are a number of things in the language which confuse this, one of them being the fact that functions (which are also objects) have a property called prototype - which does not point to the real "prototype" object which gets consulted if a nonexistent property is referenced on the function object. (A normal function foo has Function.prototype is the object that it delegates to - not foo.prototype.) However, if you declare a function foo, an object called foo.prototype is created - which is basically an empty, nondescript object. Its significance is that if the function foo is used as a "constructor" - that is, if you make an object by calling new foo() - foo.prototype will then be the object that JS will look up properties (including methods) on if any object constructed from foo happens to fail a property lookup.
This is why, at least in pre-ES6 code, you quite frequently saw this kind of pattern:
function foo(a,b) {
this.a = a;
this.b = b;
}
foo.prototype.add = function() {
this.a = this.a + this.b;
}
var myFoo = new foo(1,2);
console.log(myFoo.a);
myFoo.add();
console.log(myFoo.a);
myFoo.add();
console.log(myFoo.a);
console.log(myFoo.hasOwnProperty("add"));
Despite appearances, myFoo doesn't actually have a method add in this example - as confirmed by the final console.log. When the JS engine fails to find the property though, it goes to myFoo's "internal prototype", which happens to be foo.prototype. And that's why the method works, as it would on any object constructed from foo.
Anyway, this is leading up to the fact that Arrays, which could be (although almost never are) constructed by calling new Array (I didn't use the new operator above, but I could have done, this is a case where it makes no difference), therefore delegate to Array.prototype. All those array methods that you know and love don't "really" exist on the arrays you call them on:
var a = [1,2,3];
a.push(4);
console.log(a);
console.log(a.hasOwnProperty("push"));
console.log(Array.prototype.hasOwnProperty("push"));
So array methods only work because those methods are actually found on the Array.prototype object, to which all arrays delegate for property/method access if the lookup doesn't succeed on the array itself. (And this is why, if you look up any of them on MDN, the top of the page always says Array.prototype.<method_name>, because that's where the method "really" lives.)
A drastic demonstration of this (please DON'T do this in production code!)
// you're used to doing this, and it works:
[1,2].push(3);
console.log("that went fine");
// vandalism on a grand scale!
Array.prototype.push = undefined;
// now you can'tse the "push" method anymore!
[1,2,3].push(4);
But I'm going to end on something of an anticlimax. The above is true for array methods - but the length array property isn't a method. As observed above, it's just a "plain" (non-function) property, which "magically" behaves somewhat like a function call. As observed in the OP, .length property accesses don't delegate as the method calls shown above do, the property exists on each array in itself.
So why does Array.prototype still itself have a length property? Well, Array.prototype is actually itself an array. In fact, that's not the only thing:
Array.prototype.push(1);
console.log(Array.prototype);
Function.prototype();
notice that Array.prototype.push(1) ends up with Array.prototype being the singleton array [1]. So Array.prototype is "kind of like" the empty array (it's not exactly the same, because it has all those methods mentioned above directly accessible on the object itself, which a "normal" empty array doesn't). And with Function.prototype, although calling it didn't output anything, the fact that no TypeError was raised proves that it really is a function (it's actually a "no-op" function, like function() {}, but once again with various methods diretcly on it - the methods which every function has access to, such as .call and .bind).
Anyway, to cut the digression short, since Array.prototype is - as far as ordinary array properties are concerned, at least - an empty array, this explains why it has a length property, and why it's equal to 0.
I hope this clears things up, as well as demonstrating some of the more intriguing parts of Javascript.

The first part has already been answered, Array constructor is a function and functions have a .length property.
For the second, Array.prototype.length, it's a bit more obscur...
Array.prototype is actually an Array:
console.log(Array.isArray(Array.prototype)); // true
Array.prototype.push('hello');
console.log(Array.prototype.length); // 1
console.log(Array.prototype[0]); // "hello"
As to why is it an Array? Because specs say so:
The Array prototype object is an Array exotic objects and has the internal methods specified for such objects. It has a length property whose initial value is 0 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }..
There is also a note specifying that it is for compatibility reasons with previous versions of the specs.
As to why is was designed as being an Array? I have no really strong idea...

Disclaimer:
I have tried to show how constructor and instance works in general. But in fact, they have huge difference between different constructors.
Any constructor has been set its length with the value specified by the spec. Specially most of them are set to 1:
Number.length // 1
Object.length // 1
Function.length // 1
Array.length // 1
// etc.
Similarly,
Array.constructor.length // 1
// etc.
Like #Kaiido pointed out in the comment, you can see some constructor length is set:
Document.length // 0
Int8Array.length // 3
Date.length // 7
And you may also notice the length of the instance is undefined as they are not set. - It's out of the scope though.
let d = new Date
d.length === undefined // true
See at the bottom for relevant references.
But when you have an instance of it, then you're creating a new value of it:
typeof new Array === typeof Array.prototype // true
typeof new Function === typeof Function.prototype // true
// etc.
So, when you use an instance it has no length value set because it has no any parameters:
let arr = new Array // Array.prototype
arr.length === 0 // true
But when you use an instance with parameter, then you have the length property with the value of parameters
let arr = new Array(20)
arr.length === 20 // true
let func = function(a1,a2,a3,a4){}
func.length === 4 // true
// etc.
So now, you have been wondering why the constructor has length value equal to 1?
It's because the spec has set the value to be 1 initially.
Every built-in Function object, including constructors, has a length property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description, including optional parameters.
The value of the [[Prototype]] internal slot of the Object constructor is the intrinsic object %FunctionPrototype%.
Besides the length property (whose value is 1),
See these references:
Standard built in objects,
19.1.2 Properties of the Object Constructor,
19.2.2 Properties of the Function Constructor,
etc.
Also, you can see the prototype has length to 0, you already know it why in the preceding example.
Though, here's just a reference stating that:
19.2.3 Properties of the Function Prototype Object
And there some constructor whose length is set different. This is the out of scope of this answer. Though, here's a reference for the date constructor:
20.3.3 Properties of the Date Constructor
So, it's totally up to the spec how they have been defined.

Array.length
Array is constructor which means its type is "function". You try the checking it console.
typeof Array //"function"
According to MDN
The length property indicates the number of parameters expected by the function.
As the Array function expects single argument so Array.length = 0
array_instance.length
The length property of an object which is an instance of type Array sets or returns the number of elements in that array
As we know that arrays are actually objects so objects can have properties. The property length is on the instance of array.
Now second question you may ask why we don't get the length properties of array using Object.keys or for..in loop. The answer is because this property is not Enumerable.
let arr= [];
//this.property 'length2' will not be shown in for..in or Object.keys();
Object.defineProperty(arr,'length2',{
value:'length2 for test',
enumerable:false
})
//this property 'x' will be displayed in for..in loop and Object.keys()
Object.defineProperty(arr,'x',{
value:'length2 for test',
enumerable:true
})
console.log(Object.keys(arr)); //["x"]
Array.prototpe.length
According to the DOCS
The initial value of Array.prototype.constructor is the standard built-in Array
The Array prototype object is itself an array; its [[Class]] is "Array", and it has a length property (whose initial value is +0) constructor
Actually Array.prototype is an array. And remember array is always object. So it can have properties. The methods of the Array are stored in form of key:value. and there is no element in that array so it Array.prototype.length returns 0. If you push() some elements into it you will see it as array.
console.log(Array.prototype.length) //0
console.log(Array.isArray(Array.prototype)) //true
//adding element to array
Array.prototype.push('x')
console.log(Array.prototype.length) //1
As I explained in second parts you can hide properties of Object by setting enumerable:false. All the methods are keys of Array.prototype But now shown in for..in loops.

Array.length
For the number of properties in the array or the length property of an object which is an instance of type Array sets or returns the number of elements in that array.
Array.prototype.length
Inherited number of properties in the array. When you check Array.length you're actually checking Array.prototype.length

In Javascript what is more efficient, deleting an element or setting it to undefined explicitly

Say I have an Associative Array Foo, with key bar and value xyz.
console.log(Foo['bar']);
>> xyz
delete Foo['bar'];
console.log Foo['bar'];
>> undefined
Foo['bar'] = 'xyz';
console.log(Foo['bar']);
>> xyz
Foo['bar'] = undefined;
console.log (Foo['bar']);
>> undefined
My question is, which of the two is more efficient, do they differ in any way? Is there a scenario when I should use one over the other?
Thanks!
Results:
Thank you to everyone for helping out and showing me jsperf. Setting it to undefined appears to be (relatively) significantly faster then delete, although all the caveats pointed out below are also very interesting (in fact, I will probably be using delete a lot going forward to avoid future errors out of leftfield).

Be aware that deleting a property from an object will replace that property with one of the same name if one exists on the prototype chain.
Setting the property to null or undefined will simply mask it.

I didn't benchmark the performance of those operations (as I mentioned in a comment, just create a little benchmark on http://www.jsperf.com), but I'll lose some words on the difference.
You will always be good on deleteing properties, wheras setting them to undefined or null will let people and/or code hang, which check with the IN operator.
Like
if( 'bar' in Foo ) { }
will still return true if you set Foo.bar to undefined. It won't if you go with delete Foo.bar.

It'll make a negative performance difference in the long term as b is still considered a property after the latter assignment to undefined. For example:
var a = { b : 0 };
a.b = undefined;
a.hasOwnProperty("b");
>>> true
Same goes for the in keyword ("b" in a is true) so this will most likely hinder iteration when part of a larger object.

After some experimentation and research, I have concluded that, at least on my machine, using the delete operator is 6 times slower than assigning undefined. Despite that, it is important to remember what jAndy said about the prototype chain and property existence.
If you'd like to re preform my tests, I instantiated an array of 50,000 items. (Item count doesn't matter as long as it is big enough to show a difference between both tests.)
let arr = new Array(500000).fill(null).map(() => ({"item": 42}));
And then I timed the amount of time it took to assign each object's item property to undefined in the array against the amount of time it took to delete each object's item property in the array.
console.time("Assignment To Undefined");
arr.forEach(object => object.item = undefined);
console.timeEnd("Assignment To Undefined");
//Reset the array here...
console.time("Deletion");
arr.forEach(object => delete object.item);
console.timeEnd("Deletion");
All of my results:
Deletion: Approximately 200 - 170 ms.
Assignment: Approximately 15 - 40 ms.

Apart for benchmarking the actual action of deleting or assigning to undefined, you should also consider the future performance implication of using the object after you changed it.
I can think of two:
Once the property is deleted, accessing it will cause JavaScript to look it up in the prototype chain, whereas if the property still exists on the object (with undefined value), the prototype chain won't be searched.
Deleting the property changes the "object shape" and might hurt JavaScript optimizations.

From the link I commented:
The delete operator removes a property entirely. Setting a property to
undefined removes the value. Setting a property to null changes the
value to the null value.
Technically they are not equivalent, but in practice they are often
used to mean the same thing: that a property is unset.
Setting the object to null was the fastest.
Some good resources to understanding these operations:
Understanding delete
Understanding JavaScript’s ‘undefined’

Since deleting the property doesn't do the same thing programmatically as setting it to undefined, it really depends upon what programming outcome you want.
delete Foo['bar']; removes the bar property from the Foo object. It will not be there if someone iterates over the direct properties of Foo.
Foo['bar'] = undefined sets the property to undefined, but it still exists on the object and will still be there, but have a value of undefined.
So, if you want to get rid of the property, use delete. If you want the property to still be there, but have an undefined value, then set it to undefined.
If you really just want to know which is fastest and for some reason don't care out the programming difference, then go to jsperf.com, make yourself two comparison test cases and run the jsperf in a bunch of different browsers that are relevant to you. All performance questions should be answered with relevant real world testing.

Why does the delete keyword act opposite to expected?

In Chrome, try the following in the console. First
console = 0;
to assign the value 0 to console. Then
console // (prints `0`)
to check we have correctly overwritten console. Finally,
delete console
Surprisingly, console now holds the original Console object. In effect, the delete keyword "resurected" console, instead of exterminating it!
Is this expected behaviour? Where is this implemented in the Chromium code?

As mentioned in MDN's documentation on delete:
If the delete operator succeeds, it removes the property from the
object entirely, although this might reveal a similarly named property
on a prototype of the object.
Your delete simply unshadows native property inherited through prototype chain.
Some browsers have window inherit from native prototype and you'll have check out sources to see how property is inherited, if you really want to know that much details, but mostly they work just like JS' own.

Got it:
I've managed to prove the console is a property of the global object: just open your console and type: this.parent or window.parent. This will show a more complete list of properties and methods at your disposal. Including console: Console, about 2/3 of the way down, just below chrome: Object (interesting...:)). I thought of this when I remembered that I somehow managed to change the CSS rules of the console itself (in chrome, don't ask me how I got there, I can't remember). Bottom line: console ís a property of the window object. I think this backs up my explanation rather well.
#Randomblue: Since you're interested in how this is implemented in v8 you can check the trunk here, or browse the bleeding. Somewhere you'll find a test dir, that has a number of files that deal with delete. Special attention is given to delete used on global variables/properties: they can't be deleted, in other words: the console is never really gone. I would like to know why this answer went from being voted helpful and accepted to not-helpful and not-accepted, though...
It's perfectly simple. Console isn't some random, stand-alone, object. It's actually a property of the global object. Open your console and type this.console === console or window.console === console. It logs true, of course.
So thanks to implied globals console = 0 is pretty much the same as window.console = 0. You're sort of reassigning a property of an instance. The difference with normal objects is that the global object isn't just any old object: it's properties cannot be deleted (somewhere here on MDN). So your global is masking the console object, which is still there, you've just lost your reference too it:
var bar = window.console;
console = 12;
bar.log(console);//logs 12, bar is now an alternative reference to the console object
delete console;//unmasks the console reference
console === bar;//true
Don't, for a moment, be fooled into thinking the global object doesn't have a prototype. Just type this.constructor.name and lo and behold: Window with a capital W does appear. Another way of double checking is: Object.getPrototypeOf(this); or Object.getPrototypeOf(window);. In other words, there are prototypes to consider. Like always, the chain ends with Object.prototype:
Object.getPrototypeOf(Object.getPrototypeOf(window));
In short, there is nothing weird going on here, but the weird nature of the global object itself. It behaves as if there is some form of prototypal inheritance going on. Look at the global object as though it were set up like this:
this.prototype.window = this;//<-- window is a circular reference, global obj has no name
this.prototype.console = new Console();//this is the global object
this.hasOwnProperty(console);//false
console = 0;//implied global
When attempting to access console, JS finds the property console you've just set prior to the instance of the Console object, and happily returns its value. The same happens when we delete it, the first occurance of console is deleted, but the property higher up the prototype chain remains unchanged. The next time console is requested, JS will scan the inheritance chain and return the console instance of old. The console-object was never really gone, it was merely hidden behind a property you set yourself.
Off topic, but for completeness' sake:
There are a few more things too it than this (scope scanning prior to object/prototype chain searching), due to the special character of the global object, but this is, AFAIK, the essence of it.What you need to know is that there is no such thing (in JS) as an object without (at least) 1 prototype. That includes the global object. What you're doing merely augments the current global object's instance, delete a property and the prototype takes over again. Simple as that. That's what #Peeter hinted at with his answer: implied globals are not allowed in strict mode, because they modify the global object. Which, as I tried to explain here, is exactly what happens here.

Some properties of the window object aren't deletable. True is returned because you aren't running in strict mode. Try the following (not in console):
"use strict";
delete console;
and you will get an exception (JSFiddle).
You can read more about how this is handled at http://es5.github.com/#x11.4.1

First, this is not just the console, you can do this with every native property every browser-defined property on window.
setTimeout = 0;
setTimeout //=> 0
delete window.setTimeout;
setTimeout //=> function setTimeout() { [native code] }
Properties that are part of the ECMA-Script Spec can be fully overwritten & deleted:
Array = 0;
Array //=> 0
delete window.Array;
Array //=> ReferenceError
You can nearly overwrite any property on window, delete the overwrite and get back to the normal function.
The simple reason for this is that console and all the other native global functions browser defined properties are not linked to the DOMWindow Object via javascript but via C++. You can see the console being attached to the DOMWindow right here and the implementation of DOMWindow here
That also means that the window object is somehow a C++ Object masked as a javascript object the window object is at least partly defined by C++, and it is not prototypical inheritance doing the magic: Take for example:
window.hasOwnProperty('console') //=> true, console is defined directly on the window
window.__proto__.hasOwnProperty('console') // => false, the window prototype does not have a console property
Also, if it was prototypical inheritance, the following would lead to console returning 3:
window.__proto__.console = 3;
delete console;
console //=> still returns console;
window.hasOwnProperty('console') //=> the window still has it.
The same with a property respecting prototypical inheritance:
window.someProp = 4;
window.__proto__.someProp = 6;
someProp //=> 4
delete someProp;
someProp //=> 6
Therefore, when you set console to anything, it is gone and can only be resurrected by (hoorray for the irony): delete console.
So, what it means is that you cannot delete any native properties on the window object. Try to delete window.console when it is not overwritten, it will just pop up again. The fact that you are able to overwrite it in the first place (even in strict mode) without receiving any kind of warning (in my eyes) one of the key vulnerabilities of javascript (set setTimeout on nearly any page to 0 and see it tear itself apart), but as they say in spiderman:
With great power comes great responsibility
Update
To include a hint that this is specific to the implementation of the browser / engine and not any requirement of the language itself: In nodejs, deleting both engine-specified properties and ecma-script properties on the global object works:
delete this.console //=> true
console //=> ReferenceError
delete parseInt //=> true
parseInt //=> ReferenceError

The exact same thing happens in Firefox.
I'm assuming the following, based on observations of my own.
Variables are first checked to see if they match local variables, if not, then it will be checked to see if they match window.variable.
When you set console to 1, you set the local variable console to 1, so any lookups will see that instead of window.console (which still exists). When you delete console the local variable console gets deleted. Now any lookups of console will match window.console. That's why you get the behaviour you get.
I am assuming this based on experimenting with the JavaScript interpreter in Firefox.
And, I'm sorry about incorrect terminology (feel free to edit), I'm not that experienced with namespaces.

The delete operator removes a property from an object.
...
You can use the delete operator to delete variables declared
implicitly but not those declared with the var or the function
statement.
See delete on MDN
Edit:
See also Understanding delete if you're really into hardcore JavaScript.

What happens is you are overwriting the objects prototype, then you delete the overwritten value and what is left... is the original object, which is it's prototype.

Expected behavior. Little known fact that the Javascript console does not run in the browser's global space, but rather it runs within its own anonymous function.
I know that different browsers handle things differently, but in short -- delete does not operate as expected because it isn't operating in the global space.
If you really want to see things break, try playing with delete window.console
Ok, it is official -- I'm an idiot. One of the new features in ECMAScript is the ability to declare a property as dontdelete. Sorry about that confusion.

Javascript expando objects

What are expando objects in javascripts?
For what purpose we need this ? Any complete example will be appreciated
I found 1 article here Javascript: The red-headed stepchild of web development

Well, in javascript, any object is an expando object. What it means is, as the article covers, that whenever you try to access a property1 it will automatically be created.
var myObj = {}; // completely empty object
myObj.myProp = 'value';
The moment you assign myProp a value, the property myProp is dynamically created, eventhough it didn't exist before. In a lot of other languages, such as C#, this is not normally possible (actually C# has just enabled expando object support as well, but that's besides the point). To access a property in a normal class in C#, you need to specify in the class that it does indeed have this property.
1 Not quite correct. See npup's comment below for clarification.

Everything except primitive types(string, number,boolean) are objects and support Key:values structure. properties(keys) can be accessed and set using the dot notation as well as the square brackets.
var myObj = {};
myObj.myProp1 = 'value1'; //works, an expando property
myObj[myProp2] = 'value2'; // doesn't work, myProp2 is an undefined name.
myObj['myProp2'] = 'value2'; // works , an expando property
myObj[2010]= 'value'; //note the key is number, still works, an expando property??
myObj.2010 = 'value'; // FAILS. to use dot notation, key must be a string

An article written in 2007 that uses document.all (as the only way to access elements)? That's a big red flag.
It is just dressing up "You can add properties to an object" with some buzzwords.
We need to be able to do this because otherwise we wouldn't be able to store data, and that would make JavaScript a pretty useless language.
(Everything is an array? No it isn't. And it iterates over an object without a hasOwnProperty wrapper. That isn't safe. Just keep away from the article, it is worse than useless)

JavaScript turns elements with specific IDs of names into expandos of the returned DOM object. It is explained here.