Question

So jQuery() is is a function and document is an object which is used as a parameter of the jQuery() function.

I suppose, the jQuery(document) function then return an object, which has a ready() method? Is it correct? What kind of object does jQuery(document) return then?

Answer 1

jQuery uses a lot of magic (e.g. to avoid the need of using new).

jQuery(...) always creates a new jQuery object, containing zero or more elements (depending on the arguments) passed to it.

jQuery has a ready method so you can call it on any jQuery object. It doesn’t matter if you use $().ready() or $(document).ready() or $('whatever').ready() – it will always do the same thing:

ready: function( fn ) {
    // Attach the listeners
    jQuery.bindReady();

    // Add the callback
    readyList.add( fn );

    return this;
},

As you can see, this is only returned for chaining – the actual logic of the function does not use it at all.