Is array both associative and indexed?

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];
    }
}

Yes.

test = new Array();
test[0] = 'yellow';
test['banana'] = 0;
alert(test[test['banana']]);