Javascript array with properties?

Question

Ok, so i don't understand what this means, in chrome console's I got this :

enter image description here

Now why is the cat4 property an Array of 0 element but has properties cat5 ? Here is what i get with JSON.stringify() :

{
    "cat0": [
        {
            "category": "cat0",
              ----...other properties here ...----
        }
    ],
    "cat4": [],
    "cat7": []
}

@njzk2 No, or the other arrays would have the same property. A property has just been added to that specific array (see answer below). — Denys Séguret
– Denys Séguret, Commented Jan 15, 2014 at 14:02
@dystroy : all those arrays seem to have a "categoryName" property, but it is true that I did not see the cat5 in cat4 array — njzk2
– njzk2, Commented Jan 15, 2014 at 14:15

Inigo · Accepted Answer · 2021-06-19 20:50:31Z

3

You can give a property to an array, as it's an object :

var a = [];
a.name = "value";
console.dir(a); // you see name:value when you open the tree

but when JSON.stringify serializes it as JSON, those properties are ignored because the stringification of an array is specific : The specification says that arrays are to be stringified in JSON as the sequence of their elements (see 2.3). The actual implementation of JSON.stringify is now browser dependent but you can see the test in the old Crockford's code :

if (Object.prototype.toString.apply(value) === '[object Array]') {

edited Jun 19, 2021 at 20:50

Inigo

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answered Jan 15, 2014 at 13:55

Denys Séguret

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