For instance, if I do a[1000000]=1; will it use memory for 1000000 elements or just for this one?
+6
A:
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"
Tomalak
2010-04-04 20:52:15
Whether they're sparse or not would likely depend on the implementation. I would dare to guess that the undefined values the values inbetween get initialized to do take memory on current implementations.
Matti Virkkunen
2010-04-04 20:54:41
I would have expected them to be sparse to save memory and time in the `var a[1000000] = x` situation. But this is very likely implementation-specific, I agree. Initializing a million `undefined` values seems like a waste of resources.
Tomalak
2010-04-04 20:58:50
Implementing them as sparse arrays would make the common case (not having thousands of undefined values) needlessly inefficient.
Matti Virkkunen
2010-04-04 21:00:18
JS arrays _are_ associative.
SLaks
2010-04-04 21:00:55
No they aren't.
Matti Virkkunen
2010-04-04 21:04:36
@Matti: I would be interested in a definition/spec that clarifies this point. Thinking about it, one *could* interpret the fixed index->value connection as associativity.
Tomalak
2010-04-04 21:18:11
@Matti: Oh, and while we are at it: IMHO, the "common case" is a) declaring an array by literal or b) incrementally appending to it in a loop. Either way - where would sparseness imply inefficiency here?
Tomalak
2010-04-04 21:26:20
Arrays are Objects with string-numeric keys. Like all Object, they *are* associative. Browsers usually *also* provide an optimisation for numeric keys in an Array, but that's an internal implementation matter, not part of the language.
bobince
2010-04-04 21:42:19
@bobince: Thanks for the clarification. This makes a lot of sense.
Tomalak
2010-04-04 21:47:34
An Array's length property and member functions ignore any properties of the array aside from its integer-indexed elements. If you were to add [0], [1], and ["str"] indexes to an Array, the length would be 2 (indexes 0 and 1), and copying the array via `slice()` would give you an Array without the "str" property that you added to the original. If you need an associative array in Javascript, you should use an Object. There's no benefit to using Array instead, or of thinking of it as an associative array just because it appears to work as one superficially.
Josh Townzen
2010-04-04 21:59:34
It would seem I have failed at remembering the spec. Please excuse my stupidity.
Matti Virkkunen
2010-04-04 22:31:53
A:
JS arrays are auto-growing. Setting a[100] to 1 on an empty array will populate the first 99 elements with "undefined".
Tin
2010-04-04 20:53:06
Actually nothing gets *populated*. That's why it won't be just the first elements that'll *be undefined* (as opposed to *populated with undefined*) but all elements (except `a[100]`) will be `undefined`.
Miguel Ventura
2010-04-04 20:56:04
To demonstrate the difference: `a[1]=undefined`. Now `a.length===2` and `a['0']===undefined` and `a[1]===undefined`, *but* whilst `'1' in a` is `true`, `'0' in a` is `false`.
bobince
2010-04-04 21:38:56
+1
A:
You may use object literal as a kind of 'associative aray' in some cases:
var object = {
"first": "1",
"second": "2",
"third": "3",
"fourth": "4"
};
object.fifth = "5";
object.["sixth"] = "6";
But it has its limitations... There is no magic 'length' parameter and you will not have access to methods that every array has.
smentek
2010-04-04 21:03:10