Question

Internally, JavaScript has only objects (and primitive types). Objects are unordered collection of key:value pairs, where key is string and value can be any JavaScript type.

Arrays, though on the outside look like normal arrays of any other language, are actually a special case of objects, with natively supported language syntax.

Each array is an object where the key is a number and value can be any type. The keys are managed by JavaScript itself so that we can manipulate the array as an ordered collection of values. This is the reason we can use arrays in a for-in loop too. Additionally, JavaScript provides standard array operations (length, indexOf, splice, slice, join) as methods that take use the numbered keys to do their thing.

>>> typeof []
"object"

>>> Object.prototype.toString.call([])
"[object Array]"

Is my understanding correct, or am I missing something?

Answer 1

One thing you missed up there is that the numeric indices themselves are simply named properties of the array object, there is no underlying order to them.

Another thing that might be worth keeping in mind is that the Array constructor can be modified, which will influence all arrays from that point on.

Consider the following:

var a = new Array(1,2,3,4,5);
a.slice(1); // returns [2,3,4,5]

//Now for some prototype modification
Array.prototype.slice = function(){alert("Foo");};

var b = new Array(1,2,3,4,5);
a.slice(1); // alerts "Foo"!

//The [] syntax is also affected
var c = [1,2,3,4,5];
c.slice(1); // alerts "Foo"!