I need to create an associative array in javascript with an integer key as follows;
a["10"] = "ten";
but when i create an array, it puts the value to the 10th index of the array and it creates an array with the length 11. I want it to be a key value pair. I know this can be done by using objects but i need an array only.
JavaScript does not have associative arrays. The only way to do this in JavaScript is to use objects:
var a = {
'10': 'ten'
};
ECMAScript does have Associated Arrays1 - Objects (and by extension, Arrays) are an example
However, some properties of Arrays are treated specially:
Array objects give special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P ..
.. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index ..
Thus, given arr = [], the expressions arr["1"] and arr[1] refer to the same property name. Since P (the property name) is "1" and length is 0 from above, then assignment to such property will set arr.length to ToUint32(P)+1, or 2.
It is not possible to change this behavior. If you wish to not have a special length property, then use a "normal" Object instead of an Array. However, many of the Array.prototype functions can be used with arbitrary objects (with some implementation quirks aside) that have a length property and an Object can be created such that it uses Array.prototype as its own prototype.
All that being said, the post does not say what the real issue is. Instead of supposing that it must be done in that particular manner, consider explaining what the intent is: e.g. why a["10"]? And what is wrong if there are "11 items" if the object will be used in a List?
1 Please read the article before debating this statement: the term "Array" in the name does not imply an ordered sequence nor does it preclude an additional notion of a Length or the use of Hashing, etc. If you are going by a different definition, make sure to specify what it is and what the desired behavior is for a given operation.
Related
When using an array constructor and pushing a few values into it as well as adding named values there are some interesting differences.
let arr = []
arr.push('a')
arr.push('b')
arr.push('c')
arr.foo1 = 'bar1'
arr.foo2 = 'bar2'
arr.forEach and for of iterates only over the indexed values, while for in iterates of the indexes/keyed values.
I guess what I'm wondering is why would anyone use this?
One thing that would be cool to have from both objects and array would be a guaranteed order of key-value pairs
(not just [{key:'value'}, {key2: 'value2'] but something where you could use for of on an object where the order would be based on the order in which the keys of the object were set. I know that's not possible but just suggesting something that would be nice to gain between both objects and arrays)
but It seems as though the order of keyed values is not able to be iterated over, so the order is not guaranteed (unless I'm wrong)
Doing this seems like it is a mix between an object and an array, but why would you mix when you can just create an object or array in a common manner?
Arrays are JavaScript objects like any other that you create. (Almost) everything in JavaScript inherits its prototype from Object.
Now the specification of array is such that the indexes are basically just specifically named properties. a[1] and a['1'] are pointing the same property. The only extra thing is that you have to have a length numeric property. In words of ECMAScript language specification, "Array objects are exotic objects that give special treatment to a certain class of property names".
Now, forEach is defined as a function that will only loop through those "special" properties that an Array instance has. It starts with 0 and finishing with length - 1, and it skips undefined values. It doesn't look at other props. So you only get fields in the array.
for of is an iterator - if you have an iterable object, you basically get its iterator function and then loop over values that it gives you. And an array iterable is giving only those indexed property names. You can read a bit more over at MDN.
Once again, an array is object like any other, but we give some of it's properties special meaning, and it inherits some functions from Array prototype that can deal with those properties.
But for in does not care if this object is special or not. It will simply loop over ALL properties of any object you give it, including an array. That's why when you for in loop the object, you're looping the object's properties, and array indexes are included into this.
Note: usage of for ... in ... is not recommended. E.g in a sparse array, (where you set, e.g. a[0] = 1, a[10] = 1, and leave all other indexes undefined), a for in would just log out those two properties.
So if somebody gives you an object that has properties called 0 and 10, would you call it an array? Nope. Another bad thing is that it will also go over the object's prototype and list out the properties of all the objects this array might have inherited from - and you likely do not want that.
Short answer: there is no such thing as a "named array" in JS.
JS's datatype model is based on objects, which are key/value pair containers, and arrays are a special kind of object with additional (native) logic for dealing with numerical keys.
As such, anything you can do to an object, you can do to an array, but you shouldn't: if you need named keys, use an object. If you need numerical keys with derived properties like length and a utility API like push/pop/shift/unshift, forEach, map, etc. use an array.
Also note that the length property says nothing about the actual array footprint in memory: arrays are not like C/Java/etc arrays at all, behaving more like vectors/arraylists instead: they're just JS objects with numerical key/pair bindings, so if you set an array[0] and then you set an array[99], the length will claim "100" and that number will mean nothing at all: your array, in memory, is just an object with a key/value pair keyed on the string 0 and a key/value pair keyed on the string 100.
This quora answer is pretty good further reading, explaining all of this based on the actual ECMAScript spec definitions.
I have generally found javascript to be transparent, in that there are very few black boxes where "magic" just happens and you should just accept and look the other way, however I have not found any answer to how the Array brackets [] notation actually works under the hood.
let arr = [4, 5, 6, 7]
console.log(arr[3]) // <- How does this work?
What is javascript doing to access the item at index 3. Does it internally call some method on the Array.prototype?
With an object, the [] is a shortcut for a property accessor.
let obj = {
a: 'hello',
b: 'world'
}
obj['a'] === obj.a // true
Is an array then just an object with a long list of integer based properties?
let objArray = {
0: 'hello',
1: 'world'
}
let realArray = ['hello', 'world']
objArray[0] === 'hello' // true
realArray[0] === 'hello' // true
objArray.0 // SyntaxError: Unexpected number
realArray.0 // SyntaxError: Unexpected number
I have seen many many online discussions that all come to the conclusion that you cannot overload the brackets notation to truly subclass an Array but I have never seen an explanation on what magic is happening under the hood that allows the Array to work the way it does.
The obvious follow up question is whether there is any way to intercept the bracket notation access to define your own behavior, but I think I already know the answer to that.
You'd probably have to look at the implementation code to know precisely what's going on, but the basic idea is that arrays are actually layered atop objects.
This is backwards:
With an object, the [] is a shortcut for a property accessor.
The bracket notation is more fundamental. Thus, obj['foo'] and obj.foo work the same, but there is no equivalent for obj['foo & bar'], which is perfectly legitimate, and will respond with a value if obj has a key named "foo & bar".
Is an array then just an object with a long list of integer based properties?
Not quite, but you're not far off. Arrays are objects with the Array prototype, and with a little bit of additional magic to set the length property when new keys are added, or remove keys when that length is set.
And no, you cannot override the [] operator for your own purposes.
Is an array then just an object with a long list of integer based properties?
Yes, in it's simplest form, an Array is an Object with a list of integer base properties that is based on the Array prototype (which gives access to all the array methods like map, forEach, etc.)
As for intercepting the bracket notation, no, I have not seen anything that would allow that besides creating your own Object that has the methods you need (and then only access that object via the appropriate methods).
More info from MDN:
Arrays are list-like objects whose prototype has methods to perform traversal and mutation operations. Neither the length of a JavaScript array nor the types of its elements are fixed. Since an array's length can change at any time, and data can be stored at non-contiguous locations in the array, JavaScript arrays are not guaranteed to be dense; this depends on how the programmer chooses to use them. In general, these are convenient characteristics; but if these features are not desirable for your particular use, you might consider using typed arrays.
Arrays cannot use strings as element indexes (as in an associative array) but must use integers. Setting or accessing via non-integers using bracket notation (or dot notation) will not set or retrieve an element from the array list itself, but will set or access a variable associated with that array's object property collection. The array's object properties and list of array elements are separate, and the array's traversal and mutation operations cannot be applied to these named properties.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Indexed_collections#Array_object
Is it bad to use code like:
var a = [1,2,3,4];
a.length = 2; // 3 and 4 are removed
Does it have decent browser support? Do the removed values get garbage collected properly?
Does it have decent browser support?
Yes. This has been present since the very first edition of ECMAScript:
Specifically, whenever a property is added whose name is an array index, the
length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever
the length property is changed, every property whose name is an array index whose value is not smaller than the
new length is automatically deleted.
Standard ECMA-262, 15.4
In ECMAScript 5, this was moved to 15.4.5.2.
Attempting to set the length property of an Array object to a value that is numerically less than or equal to the largest numeric property name of an existing array indexed non-deletable property of the array will result in the length being set to a numeric value that is one greater than that largest numeric property name.
Standard ECMA-262, 15.4.5.2
All browsers support this behavior.
Do the removed values get garbage collected properly?
Yes. (See quotes above, and 15.4.5.1.)
Is it an antipattern to set array length in Javascript?
No, not really. While arrays are "exotic objects" in ES6-speak, the real crazily unique thing about arrays are their indexing, not setting the length property. You could replicate the same behavior with a property setter.
It's true that property setters with non-subtle effects are somewhat unusual in JS, as their side-effects can be unobvious. But since .length has been there in Javascript from Day 0, it should be fairly widely understood.
If you want an alternative, use .splice():
// a is the array and n is the eventual length
a.length = n;
a.splice(n, a.length - n); // equivalent
a.splice(n, a.length); // also equivalent
If I were avoid setting .length for some reason, it would be because it mutates the array, and I prefer immutable programming where reasonable. The immutable alternative is .slice().
a.slice(0, n); // returns a new array with the first n elements
Arrays are exotic objects.
An exotic object is any form of object whose property semantics differ in any way from the default semantics.1
The property semantics for arrays are special, in this case that changing length affects the actual contents of the array.
An Array object is an exotic object that gives special treatment to array index property keys [...] 2
The behavior of the specific property key for this array exotic object is outlined as well.
[...] Every Array object has a length property whose value is always a nonnegative integer less than 2^32. The value of the length property is numerically greater than the name of every own property whose name is an array index; whenever an own property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant [...]
This section is detailing that the length property is a 32 bit integer. It is also saying that if an array index is added, the length is changed. What this is implying is that when an own property name that is not an index is used, the length is not changed and also that names which are not indexes are considered numerically less than the indexes. This means that if you have an array and also add a string (not implicitly numeric either, as in not "3") property to it, that changing the length to delete elements will not remove the value associated with the string property. For example,
var a = [1,2,3,4];
a.hello = "world";
a.length = 2; // 3 and 4 are removed
console.log(a.hello);//"world"
[...] Specifically, whenever an own property is added whose name is an array index, the value of the length property is changed, if necessary, to be one more than the numeric value of that array index; [...]
In addition to the truncation, expansion is also available by use of an index. If an index value is used (an integer basically) then the length will be updated to reflect that change. As arrays in JavaScript are sparse (as in, no gaps allowed) this means that adding a value for a larger index can make an array rather larger.
var a = [1,2,3,4];
a[50] = "hello";
console.log(a.length);//51
[...] and whenever the value of the length property is changed, every own property whose name is an array index whose value is not smaller than the new length is deleted.
Finally, this is the specific aspect in question which the OP raises. When the length property of an array is modified, it will delete every index and value which is numerically more than the new length value minus 1. In the OP's example, we can clearly see that changing the length to 2 removed the values at index 2 and 3, leaving only the first two values who had index 0 and 1.
The algorithm for the deletion mentioned above can be found in the ArraySetLength(A, Desc) definition.
b. Let deleteSucceeded be A.[[Delete]](ToString(oldLen)). 3
Which is converting the index to a string and using the delete behavior for objects on the index. At which point the entire property is removed. As this internally reaches the delete call, there is no reason to believe that it will leak memory or even that it is an anti pattern as it is explicitly described in the language specification.
In conclusion, Does it have decent browser support? Yes. Do the removed values get garbage collected properly? Yes.
However, is it an anti-pattern? This could be argued either way. What it really breaks down to is the level of familiarity with the language of others who would be using the same code which is a nice way of saying that it may not have the best readability. On the other hand, since JavaScript does take up bandwidth or memory with each character used (hence the need for minification and bundling) it can be useful to take this approach to truncate an array. More often than not, worrying about this type of minutia is going to be a micro-optimization and the use of this special property should be considered on a case to case basis in context with the overall design of the related code.
1. The Object Type ECMA 6
2. Array Exotic Objects ECMA 6
3. ArraySetLength(A, Desc) ECMA 6
As it is writeable https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/length (seen here) It should be okay to use it that way
You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements does not increase; for example, if you set length to 3 when it is currently 2, the array still contains only 2 elements.
There's even an example how to shorten an array, using this on Mozilla Developer Network
if (statesUS.length > 50) {
statesUS.length = 50;
}
It's better to use slice because your intentions are clear.
var truncated = a.slice(0,2)
Slice does create a new array though.
If that is an issue for you then there is nothing wrong with modifying the length property.
It's actually the fastest way of truncation and supported in all browsers. jsperf
Is it an antipattern to set array length in JavaScript?
No. It would be OK to set it to truncate the array.
Does it have decent browser support?
Yes, see the compatiblity matrix here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/length
Do the removed values get garbage collected properly?
Yes, they should. Will they actually get gced - you will need to profile the JavaScript code if this is a real concern.
This way of truncating an array is mentioned in JavaScript: Good Parts, so it is not an anti-pattern.
It is not an anti pattern. It is supported in all major browsers.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/length
I had a generic question about JavaScript arrays. Are array indices in JavaScript internally handled as strings?
I read somewhere that because arrays are objects in JavaScript, the index is actually a string. I am a bit confused about this, and would be glad for any explanation.
Formally, all property names are strings. That means that array-like numeric property names really aren't any different from any other property names.
If you check step 6 in the relevant part of the spec, you'll see that property accessor expressions are always coerced to strings before looking up the property. That process is followed (formally) regardless of whether the object is an array instance or another sort of object. (Again, it just has to seem like that's what's happening.)
Now, internally, the JavaScript runtime is free to implement array functionality any way it wants.
edit — I had the idea of playing with Number.toString to demonstrate that a number-to-string conversion happens, but it turns out that the spec explicitly describes that specific type conversion as taking place via an internal process, and not by an implicit cast followed by a call to .toString() (which probably is a good thing for performance reasons).
That is correct so:
> var a = ['a','b','c']
undefined
> a
[ 'a', 'b', 'c' ]
> a[0]
'a'
> a['0']
'a'
> a['4'] = 'e'
'e'
> a[3] = 'd'
'd'
> a
[ 'a', 'b', 'c', 'd', 'e' ]
Yes, technically array-indexes are strings, but as Flanagan elegantly put it in his 'Definitive guide':
"It is helpful to clearly distinguish an array index from an object property name. All indexes are property names, but only property names that are integers between 0 and 232-1 are indexes."
Usually you should not care what the browser (or more in general 'script-host') does internally as long as the outcome conforms to a predictable and (usually/hopefully) specified result. In fact, in case of JavaScript (or ECMAScript 262) is only described in terms of what conceptual steps are needed. That (intentionally) leaves room for script-host (and browsers) to come up with clever smaller and faster way's to implement that specified behavior.
In fact, modern browsers use a number of different algorithms for different types of arrays internally: it matters what they contain, how big they are, if they are in order, if they are fixed and optimizable upon (JIT) compile-time or if they are sparse or dense (yes it often pays to do new Array(length_val) instead of ninja []).
In your thinking-concept (when learning JavaScript) it might help to know that arrays are just special kind of objects. But they are not always the same thing one might expect, for example:
var a=[];
a['4294967295']="I'm not the only one..";
a['4294967296']="Yes you are..";
alert(a); // === I'm not the only one..
although it is easy and pretty transparent to the uninformed programmer to have an array (with indexes) and attach properties to the array-object.
The best answer (I think) is from the specification (15.4) itself:
Array Objects
Array objects give special treatment to a certain class of property
names. A property name P (in the form of a String value) is an array
index if and only if ToString(ToUint32(P)) is equal to P and
ToUint32(P) is not equal to 232−1. A property whose property name is
an array index is also called an element. Every Array object has a
length property whose value is always a nonnegative integer less than
232. The value of the length property is numerically greater than the name of every property whose name is an array index; whenever a
property of an Array object is created or changed, other properties
are adjusted as necessary to maintain this invariant. Specifically,
whenever a property is added whose name is an array index, the length
property is changed, if necessary, to be one more than the numeric
value of that array index; and whenever the length property is
changed, every property whose name is an array index whose value is
not smaller than the new length is automatically deleted. This
constraint applies only to own properties of an Array object and is
unaffected by length or array index properties that may be inherited
from its prototypes.
An object, O, is said to be sparse if the following algorithm returns
true:
Let len be the result of calling the [[Get]] internal method of O with argument "length".
For each integer i in the range 0≤i<ToUint32(len)
a. Let elem be the result of calling the [[GetOwnProperty]] internal method of O with argument ToString(i).
b. If elem is undefined, return true.
Return false.
Effectively the ECMAScript 262 spec just ensures to the JavaScript-programmer unambiguous array-references regardless of getting/setting arr['42'] or arr[42] up to 32-bit unsigned.
The main difference is for example (auto-updating of) array.length, array.push and other array-sugar like array.concat, etc.
While, yes, JavaScript also lets one loop over the properties one has set to an object, we can not read how much we have set (without a loop). And yes, to the best of my knowledge, modern browsers (especially chrome in what they call (but don't exactly specify)) 'small integers' are wicked fast with true (pre-initialized) small-int arrays.
Also see for example this related question.
Edit: as per #Felix Kling's test (from his comment above):
After arr[4294967294] = 42;, arr.length correctly shows 4294967295. However, calling arr.push(21); throws a RangeError: Invalid array length. arr[arr.length] = 21 works, but doesn't change length.
The explanation for this (predictable and intended) behavior should be clear after this answer.
Edit2:
Now, someone gave the comment:
for (var i in a) console.log(typeof i) shows 'string' for all indexes.
Since for in is the (unordered I must add) property iterator in JavaScript, it is kind of obvious it returns a string (I'd be pretty darned if it didn't).
From MDN:
for..in should not be used to iterate over an Array where index order
is important.
Array indexes are just enumerable properties with integer names and
are otherwise identical to general Object properties. There is no
guarantee that for...in will return the indexes in any particular
order and it will return all enumerable properties, including those
with non–integer names and those that are inherited.
Because the order of iteration is implementation dependent, iterating
over an array may not visit elements in a consistent order. Therefore
it is better to use a for loop with a numeric index (or Array.forEach
or the for...of loop) when iterating over arrays where the order of
access is important.
So.. what have we learned? If order is important to us (often is with arrays), then we need this quirky array in JavaScript, and having a 'length' is rather useful for looping in numerical order.
Now think of the alternative: Give your objects an id/order, but then you'd need to loop over your objects for every next id/order (property) once again...
Edit 3:
Someone answered along the lines of:
var a = ['a','b','c'];
a['4'] = 'e';
a[3] = 'd';
alert(a); // returns a,b,c,d,e
Now using the explanation in my answer: what happened is that '4' is coercible to integer 4 and that is in the range [0, 4294967295] making it into a valid array index also called element. Since var a is an array ([]), the array element 4 gets added as array element, not as property (what would have happened if var a was an object ({}).
An example to further outline the difference between array and object:
var a = ['a','b','c'];
a['prop']='d';
alert(a);
see how it returns a,b,c with no 'd' to be seen.
Edit 4:
You commented: "In that case, an integer index should be handled as a string, as it is a property of the array, which is a special type of JavaScript object."
That is wrong in terms of terminology because: (strings representing) integer indexes (between [0, 4294967295]) create array indexes or elements; not properties.
It's better to say: Both an actual integer and a string representing an integer (both between [0, 4294967295]) is a valid array index (and should conceptually be regarded as integer) and creates/changes array elements (the 'things'/values (only) that get returned when you do arr.join() or arr.concat() for example).
Everything else creates/changes a property (and should conceptually be regarded as string).
What the browser really does, usually shouldn't interest you, noting that the simpler and clearer specified you code, the better chance the browser has to recognize: 'oh, let’s optimize this to an actual array under the hood'.
Let's see:
[1]["0"] === 1 // true
Oh, but that's not conclusive, since the runtime could be coercing "0" to +"0" and +"0" === 0.
[1][false] === undefined // true
Now, +false === 0, so no, the runtime isn't coercing the value to a number.
var arr = [];
arr.false = "foobar";
arr[false] === "foobar" // true
So actually, the runtime is coercing the value to a string. So yep, it's a hash table lookup (externally).
In JavaScript there are two type of arrays: standard arrays and associative arrays (or an object with properies)
[ ] - standard array - 0 based integer indexes only
{ } - associative array - JavaScript objects where keys can be any strings
So ...
var arr = [ 0, 1, 2, 3 ];
... is defined as a standard array where indexes can only be integers. When you do arr["something"] since something (which is what you use as index) is not an integer you are basically defining a property to the arr object (everything is object in JavaScript). But you are not adding an element to the standard array.
Crockford writes in http://javascript.crockford.com/survey.html:
"There are two ways to make a new array:
var myArray = [];
var myArray = new Array();"
So I'm confused by these two lines in some AJAX code I am reading:
var obj={}; // obj is an Object object (i.e. a hash table)
obj[4] = 'x'; // now obj is suddenly an Array object via an integer key?
In JavaScript are an object and an array really just the same thing, but with a variant on the key type?
In other words, is this the same as in php where we can use either a name (string) or an integer for a hash key?
I've Googled for an answer on this but can't seem to nail down an article which discusses this issue.
One possibility that comes to mind is that perhaps the first line is syntactic lint because the 2nd line overwrites the previous definition of obj as it creates a new Array object.
it does not become an array, it is simply an Object with a '4' property, like this:
var obj = {
'4': 'x'
};
it is just converted to a string when used as a property like obj['4'] = 'x';
Everything but primitive datatypes is an object in JavaScript. Objects can have a properties and there are two ways to access object properties:
Dot notation, foo.bar, which you can use as long as the property name is a valid identifier.
Bracket notation, foo['bar'] which you have to use if the key is not a valid identifier [spec]. For example, if it is a number, or contains a space or you have a variable with the name.
Hence, bracket notation is not a characteristic of arrays and if you see it, it does not mean the value is an array. It is simple one of two ways of accessing properties.
The elements of an array are just properties with numeric keys. Arrays are built on top of objects and implement some additional methods which treat these numeric properties in a special way. For example the .length property is automatically updated when you add new elements. But ultimately they are just normal properties.
In your example you have a simple object. You have to access the property with obj[4] or obj['4'] because obj.4 is invalid since 4 is not a valid identifier (basically everything that you can use as variable name is a valid identifier. var 4 = 'foo'; is invalid).
And since arrays are just objects, if you could use numbers as identifiers, you were also able to access an element with arr.4.
As far as I know, no, an object can't be coerced into an array. But, it can look and act like an array, and that's what's happening here. Numbers, and anything else that can be coerced to a string, are perfectly valid property names for Javascript objects, so
obj[4] = 1;
obj['spam'] = 2;
are both valid ways of setting a property on the object. That doesn't make the object an array. An Array is a special class of object with specific methods (.slice(), .concat(), etc) and a length property that's kept up to date with the number of items in the array.
Yes
Javascript Array is very different from tradition array, you can think of it as object.
var array = [1,2,3] is equivalent to var object = {'0' : 1, '1' : 2, '2' : 3}
except array inherited from Array.prototype and object inherited from Object.prototype, where Array.prototype will contain method such as length.
Javascript is a loosely-typed, prototype-based language. Even primitive types like a boolean can be treated like an object (though you aren't going to get far). Almost everything in javascript is, at root, an object.
Understanding this, an array IS an object. You can arbitrarily add properties to any object:
var xml = new XMLHttpRequest();
xml[4] = 'x';
console.log(xml);
That object is still an instance of XMLHttpRequest. It now has a property labeled 4 with a value of x. You can treat anything like this -- even a function:
var test_func = function () {
alert('woah!');
}
test_func[4] = 'x';
console.log(test_func[4]);
The take-away here is that the obj[key] = value notation is NOT indicative of an "array" type, like it is in languages such as PHP. Rather, it is an alternate way to access properties of any object, and is equivalent to obj.key = value (you can't use obj.4 = 'x', though, that's invalid syntax). The other take-away is that any object in javascript can be modified or used in pretty much any way. You shouldn't misuse objects, but you can
Check it out here: http://jsfiddle.net/w2AqJ/
Documentation
Array on MDN - https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array
Javascript "associative arrays" considered harmful by Andrew Dupont - http://andrewdupont.net/2006/05/18/javascript-associative-arrays-considered-harmful/