Are JavaScript arrays associative?

Question

For instance, if I do a[1000000]=1; will it use memory for 1000000 elements or just for this one?

Answer 1

In the ECMAScript standard (§15.4), the only special thing about array is that the length property is automatically updated (and a bunch of Array-specific prototype functions):

Array objects give special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2³²−1.
...
Every Array object has a length property whose value is always a nonnegative integer less than 2³². The value of the length property is numerically greater than the name of every property whose name is an array index; ...

Other than that, an Array is just an Object, which means it can be treated as an associative array, although you shouldn't.

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Nowadays the JS engines should detect whether the array is dense or very sparse and switch between using a linear or associative array internally. In your case, the JS engine won't allocate a million elements.

Answer 2

Would 1,000,000 elements be created?

No, arrays are sparse, but their index will be persistent. EDIT: Actually, their sparseness would be implementation-specific, but keeping them sparse in case of a[1000000] = 1 would seem a logical thing to me.

var a = [1, 2, 3, 4];
var x = a[1]; // -> x := 2

delete a[1];
var y = a[1]; // -> y := undefined

a[9] = 10;
var y = a[8]; // -> z := undefined

Are JS arrays associative?

JavaScript arrays are a subset of associative arrays (in that indices have to be integers, as shown in KennyTM's answer. JavaScript objects are fully associative:

var o = { "key1": "value1", "key2": "value2" };
var i = "key2";
var v = o[i]; // -> v := "value2"