Javascript - Associative Array copy

Question

Technically there are no such things as an Associative Array in Javascript. But for reasons I can't avoid, I've ended up having t do things such as

var x = [1,2,3];
x.someRequiredProperty = 'some value';

As expected, Javascript being Javascript, it works. But now, how do I make a copy of this 'array' so that I can work on multiple instances of this array without compromising the original?

I've tried jQuery $.extend([], x), which doesn't really give me a new copy. Is there something I'm missing?

Technically there are such things as associative arrays in javascript: {}. — davin
– davin, Commented Jul 11, 2012 at 9:16
@davin: technically they don't even have non-tricky ways of counting elements :-) — zerkms
– zerkms, Commented Jul 11, 2012 at 9:20
and I suppose you want the array methods to work on the new object as well? — nbrooks
– nbrooks, Commented Jul 11, 2012 at 9:21
@zerkms, that's not usually a requirement in the definition (cf. CLRS). Not to mention that Object.keys({...}).length isn't all that tricky. — davin
– davin, Commented Jul 11, 2012 at 9:24
@nbrooks - the array methods do work. And it seems I underestimated the $.extend - apparently if you deep copy the array, it works as advertised — Jibi Abraham
– Jibi Abraham, Commented Jul 11, 2012 at 9:27

Hubro · Accepted Answer · 2012-07-11 10:14:49Z

1

I don't like the idea of an "associative" Javascript array, since adding properties to an array won't make sense to JavaScript. For instance, the length property will only count the indexed elements (not the properties), but for(var x in assoc_array) will iterate across indexes of the array as well as the "keys". So to loop just the array items you'd have to use a full for loop for(var i = 0; i < assoc_array.length; i++)....

Anyway, a copy function like this will work for your purposes:

function copy_assoc(arr)
{
    var out = [];

    for(var key in arr)
    {
        if(!arr.hasOwnProperty(key))
        {
            continue;
        }

        out[key] = arr[key];
    }

    return out;
}

Here's a demonstration of it being used: http://jsfiddle.net/DbVV8/3/

edited Jul 11, 2012 at 10:14

answered Jul 11, 2012 at 9:30

Hubro

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Jamiec · Accepted Answer · 2012-07-11 09:22:32Z

0

First off, if you want an associative array you should use an object:

var x = {};
x[0] = 1;
x[1] = 2;
x[2] = 3;
x.someProperty = 'Foo';

Then if you want a copy you could use jQuery's each function to iterate over each property and build a copy:

var copy = {};
$.each(x,function(i,e){
   copy[i] = e;
});

Live example: http://jsfiddle.net/KhBC9/

answered Jul 11, 2012 at 9:22

Jamiec

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4 Comments

Jibi Abraham Over a year ago

Thank you, but I really do need an array, Besides, why the $each? $extend is better

Jamiec Over a year ago

You realise in JS there is pretty much no difference between an object and an array? Plus why would you use extend when you're not extending the array, just copying it?

Jibi Abraham Over a year ago

There may not be much difference, but since I'm using this to plot charts with d3.js, it has to be an array. The extra properties need to be there for other sections of the code to identify sets and so on

Hubro Over a year ago

An array has the length property, among other things. It will only count the numerical indexes though

Community · Accepted Answer · 2017-05-23 12:12:28Z

0

Let me answer my own question - For all my purposes - a deep clone of the array using $.extend(true, [], myArray) gives me a new array with all my properties intact. But I like @CodeMonkey's answer.

edited May 23, 2017 at 12:12

CommunityBot

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answered Jul 11, 2012 at 9:41

Jibi Abraham

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