Initialize an array whose indexes are scattered over integer range

You don't have to pre-define the size of an array before you assign to it. For example:

var _array = [];
    _array[0] = "foo";
    _array[1000] = "bar"; // _array.length => 1001 

_array[1] //undefined

No need to initialise the appropriate number of array elements before you assign to them.

Update

It already has been pointed out that you can use an object rather than an array. However, if you want to take advantage of array methods then this is still possible. Let me give you an example:

var obj = {
    0: 15,
    1: 10,
    2: 5,
    length: 3
};

If the object contains a length property then it can be treated as an array-like object. Although you can't call array methods directly from these objects you can use array methods.

Array.prototype.join.call( obj ); // 15,10,5

In fact using the ECMAScript 5 map function you can easily convert the above object to an array.

var _array = Array.prototype.map.call( obj, function( x ) { return x; } );

The map function does not exist in all browsers but you can use the following function if it doesn't.

Array.map = Array.map || function(a, f, thisArg) {
    return Array.prototype.map.call(a, f, thisArg); 
}

To create an array with a set number of indexes you can use

// Creates an array with 12 indexes
var myArray = new Array(12);

This isn't needed in javascript due to the way its array's work. There isn't an upper-bound for arrays. If you try to reference an item index in the array that doesn't exist, undefined is returned but no error is thrown

To create an array with perscribed indexes you can use something like array['index'] = value though this would force you to use multiple statements. Javascript doesn't have an array initalizer to allow for you to specify indexes and values all in a single statement though you can create a function to do as such

function indexArray(param) {
    var a = [], i;
    for (i=0; i<param.length; i+=1) {
        a[param[i].index] = param[i].value;
    }
    return a;
}

var myArray = indexArray([
    { index: 123456, value : "bananas" },
    { index: 12, value : "grapes" },
    { index: 564, value : "monkeys" }
]);

var array1 = []
array1[23454] = 2

Just doing this should be fine. There's no set array size for javascript in the way there is for java.

If you really want to do this all in a single statement, you can make an object instead like this:

var object1 = {
    "23454":2,
    "123":1,
    "50":3
};

and then retrieve the numbers like this:

object1["23454"]  //2

I don't really recommend this though. The array method is a cleaner way of doing it even if it takes multiple lines since it doesn't require string conversion. I don't know enough about how these are implemented in browsers to comment on the performance impact.

Update

Since the 1 line requirement is based on something being passed to the server, I would recommend passing a JSON object to the server in the form:

"{"23454":2,"123":1,"50":3}"

then this code will parse it to an object:

var object1 = JSON.parse(jsonstringfromserver);

and if you like you can always convert that to an array by enumerating over the properties with a for in loop:

var array1 = []
for ( num in object1){
    array1[num] = object1[num];

That is probably unnecessary though since object1[123] will already return 1. You only need this if you plan on doing array specific operations.

You can do what you want with an Object in this way:

var o = {23454: 'aaaa', 23473: 'bbb'};

You will lose the array methods/fields, e.g. length, but you will gain what you said you are looking for, and you will be able to add/remove members easily.