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').valuemap[2].value