Javascript - how are array stored internally?

Question

In the fiddle - http://jsfiddle.net/vwwkf18c/ or below code snippet -

var a = [3, 4];
var b = [6, 2];
var c = $.extend({}, a, b);
alert(c[1]); //alerts 2
alert(a); //alerts array a contents
alert(c); //does not return contents of c

My questions - 1) After what has been alerted, we can infer that "c" is an object but not an array object. Please confirm. 2) Secondly it is said that internal representation of an array is an object literal, is that right? Which means that array "a" would be stored as given below -

var a = {
0: 3,
1: 4
}

Is it right? 3) How is a or b stored internally and how is it different from the internal representation of "c"?

$.extend is a jquery function that expects type Object to be merged together .... — john Smith
– john Smith, Commented Jan 9, 2015 at 19:22
alert is not a debugging tool, if you learn to use the console, it will show things more clearly. $.extend creates objects. — adeneo
– adeneo, Commented Jan 9, 2015 at 19:22
In this situation, c is now: Object { 0: 6, 1: 2 }. That being said, this question isn't really the right fit for SO. Did you have a specific programming question or just theoretical stuff? — Tim Lewis
– Tim Lewis, Commented Jan 9, 2015 at 19:23
as @adeneo said, please debug via console.log(), not alert(). — Andrew Dunai
– Andrew Dunai, Commented Jan 9, 2015 at 19:24

Quentin · Accepted Answer · 2015-01-09 19:24:57Z

1

See the docs: "Returns: Object"
No. An object literal is a piece of JavaScript syntax for creating objects with. Arrays are an object type that inherits (along the prototype chain) from basic Objects. The Array type has a different toString method than the basic Object, which is why alert gives different results.
That is implementation specific (and also of no importance to anyone writing JavaScript rather than a JavaScript runtime)

answered Jan 9, 2015 at 19:24

Quentin

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1 Comment

user1329482 Over a year ago

Corollary to #2: JS Arrays are Objects. Arrays have a syntax which allows numerical indexing, which pure Objects don't. The likelihood is that an Array actually has fields arr.0, arr.1, arr.2, etc., but as #3 points out, it's implementation dependent and you shouldn't be in there. Extending Arrays using jQuery is not advisable.

Vikram · Accepted Answer · 2016-02-14 01:12:58Z

0

Yes, c is an object, but not an array.
The internal representation of a (which is an array) has a property called 'length' in addition to '0' and '1'
c has the properties of '0' and '1', but not the property 'length'

Check out this jsfiddle

var a = [3, 4];

var b = [6, 2];

var c = $.extend({}, a, b);

alert(c[1]); //alerts 2
alert(a); //alerts array a contents
alert(Object.getOwnPropertyNames(c)); //does not return contents of c
alert(Object.getOwnPropertyNames(a));

Hope that is helpful

Vikram

answered Feb 14, 2016 at 1:12

Vikram

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