Question

I am trying to understand how jQuery is coded.

They have an object:

jQuery.fn = {
     //key value pairs
}

But if I type jQuery.fn in the browser console, it just returns [] and not the object itself. Does anyone know why?

Answer 1

jQuery.fn simply meets the requirements of being array-like for the developer consoles. It’s not actually an Array instance*, but it has an interface that affords being treated as an array.

_{*If jQuery.fn were actually an array, jQuery.fn instanceof Array would evaluate to true; it doesn’t. It does copy some of the Array.prototype methods though.}

---

If you want to check if an object is actually an Array, there are two means, the simplest is obj instanceof Array, however this will be true for objects that inherit from Array. If you want to check that an object is an Array, but need to exclude objects that inherit from Array you should use:

function isArray(arg) {
    return Object.prototype.toString.call(arg) === '[object Array]';
}

Example:

var a, b;
function Foo() {}
Foo.prototype = [];
a = new Foo();
b = [];
a instanceof Array; //true
b instanceof Array; //true
isArray(a);         //false
isArray(b);         //true