Is array both associative and indexed?

There are no such things as associative arrays in Javascript. You can use object literals, which look like associative arrays, but they have unordered properties. Regular Javascript arrays are based on integer indexes, and can't be associative.

For example, with this object:

var params = {
    foo: 1,
    bar: 0,
    other: 2
};

You can access properties from the object, for example:

params["foo"];

And you can also iterate over the object using the for...in statement:

for(var v in params) {
    //v is equal to the currently iterated property
}

However, there is no strict rule on the order of property iteration - two iterations of your object literal could return the properties in different orders.

After reading the Wikipedia definition of associative array, I'm going to break with traditional JavaScript lore and say, "yes, JavaScript does have associative arrays." With JavaScript arrays, you can add, reassign, remove, and lookup values by their keys (and the keys can be quoted strings), which is what Wikipedia says associative arrays should be able to do.

However, you seem to be asking something different--whether you can look up the same value by either index or key. That's not a requirement of associative arrays (see the Wikipedia article.) Associative arrays don't have to give you the ability to get a value by index.

JavaScript arrays are very closely akin to JavaScript objects.

  arr=[];
  arr[0]="zero";
  arr[1]="one";
  arr[2]="two";
  arr["fancy"]="what?";

Yes, that's an array, and yes, you can get away with non-numeric indices. (If you're curious, after all this, arr.length is 3.)

In most cases, I think you should stick to numeric indices when you use arrays. That what most programmers expect, I think.

The link is to my blog post about the subject.

Native JS objects only accept strings as property names, which is true even for numeric array indices; arrays differ from vanilla objects only insofar as most JS implementations will store numerically indexed properties differently (ie in an actual array as long as they are dense) and setting them will trigger additional operations (eg adjustment of the length property).

If you're looking for a map which accepts arbitrary keys, you'll have to use a non-native implementation. The script is intended for fast iteration and not random-access by numeric indices, so it might nor be what you're looking for.

A barebones implementation of a map which would do what you're asking for could look like this:

function Map() {
    this.length = 0;
    this.store = {};
}

Map.prototype.get = function(key) {
    return this.store.hasOwnProperty(key) ?
        this.store[key] : undefined;
};

Map.prototype.put = function(key, value, index) {
    if(arguments.length < 3) {
        if(this.store.hasOwnProperty(key)) {
            this.store[key].value = value;
            return this;
        }

        index = this.length;
    }
    else if(index >>> 0 !== index || index >= 0xffffffff)
        throw new Error('illegal index argument');

    if(index >= this.length)
        this.length = index + 1;

    this[index] = this.store[key] =
        { index : index, key : key, value : value };

    return this;
};

The index argument of put() is optional.

You can access the values in a map map either by key or index via

map.get('key').value
map[2].value

The order in which objects appear in an associative javascript array is not defined, and will differ across different implementations. For that reason you can't really count on a given associative key to always be at the same index.

EDIT:

as Perspx points out, there aren't really true associative arrays in javascript. The statement foo["bar"] is just syntactic sugar for foo.bar

If you trust the browser to maintain the order of elements in an object, you could write a function

function valueForIndex(obj, index) {

    var i = 0;
    for (var key in obj) {

        if (i++ == index)
            return obj[key];
    }
}

var stuff = [];
stuff[0] = "foo";
stuff.bar = stuff[0]; // stuff.bar can be stuff["bar"] if you prefer
var key = "bar";
alert(stuff[0] + ", " + stuff[key]); // shows "foo, foo"

Although I agree with the answers given you can actually accomplish what you are saying with getters and setters. For example:

var a = [1];
//This makes a["blah"] refer to a[0]
a.__defineGetter__("blah", function(){return this[0]});
//This makes a["blah"] = 5 actually store 5 into a[0]
a.__defineSetter__("blah", function(val){ this[0] = val});

alert(a["blah"]); // emits 1
a["blah"] = 5;
alert(a[0]); // emits 5

Is this what you are looking for? i think theres a different more modern way to do getters and setters but cant remember.

The latest MDN documentation makes it quiet clear that Array index must be integers. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

let arr=[];
 arr[0]="zero";
 arr[1]="one";
 arr[2]="two";
 arr["fancy"]="what?";

//Arrays cannot use strings as element indexes (as in an associative array) but must use integers. 
//Setting non-integers using bracket notation will not set an element to the Array List itself
//A non-integer will set a variable associated with that ARRAY Object property collection 
let denseKeys = [...arr.keys()];
console.log(denseKeys);//[ 0, 1, 2 ]
console.log("ARRAY Keys:"+denseKeys.length);//3


let sparseKeys = Object.keys(arr);
console.log(sparseKeys);//[ '0', '1', '2', 'fancy' ]
console.log("Object Keys:"+sparseKeys.length);//4

const iterator = arr.keys();
for (const key of iterator) {
  console.log(key);//0,1,2
}