Javascript: multi-dimensional object

Question

I want to create an object, starting from something like:

var map = {};

Then, I want to add items with this function:

add = function(integerA, objectB) {
    map[objectB.type][integerA] = objectB;
}

So, this is a random example of the object structure I want to achieve:

map = {
    'SomeType' : { 0 : 'obj', 2 : 'obj', 3 : 'obj' },
    'OtherType' : { 0 : 'obj', 5 : 'obj' },
};

Now, my problem. I can't do map[objectB.type][integerA] = objectB; because map[objectB.type] is not defined. I could solve this by checking if map[objectB.type] exists through an if-statement and create map[objectB.type] = {}; when necessary.

Otherwise I could pre-load all object types. However I would prefer not to have to do this.

My question: is there a way I can create the object 'on the fly' without having to check if the type already exists every time I want to call the add function or to pre-load all the types?

It is important that my add function is so fast as possible and that the map object is correct, because I need to read and write a lot in a small amount of time (it's an animation / game application).

Answer 1

No, there is no any other way to create objects on the fly. Only check for existence every time:

add = function(integerA, objectB) {
    if (!map[objectB.type]) {
        map[objectB.type] = {};        
    }
    map[objectB.type][integerA] = objectB;
}

If you want to improve performance you might consider some caching technics.

Answer 2

You can use the boolean OR shortcut (which avoids at least an explicit if). It might not be that readable though:

var data = map[objectB.type] || (map[objectB.type] = {});
data[integerA] = objectB;

This works because an assignment actually returns the value that was assigned and an OR expression returns the first value that evaluates to true.

I don't think using an if has any impact on the performance though (actually, the way in my answer might be even "slower").

Answer 3

If you use the map only for lookups and you don't need to iterate over the dimensions, you could merge your dimensions into a single key. For example:

add = function(integerA, objectB) {
    var key = objectB.type + '-' + integerA;
    map[key] = objectB;
}