Factory function $

    the $() function removes the need to do a for loop to access a group of elements since whatever we put inside the parentheses will be looped through automatically and stored as a jQuery object. We can put just about anything inside the parentheses of the $() function.

In jQuery, the dollar sign $ is simply shorthand for jQuery. Because a $()
function is very common in JavaScript libraries, conflicts could arise if more
than one of these libraries is being used in a given page. We can avoid such
conflicts by replacing every instance of $ with jQuery in our custom jQuery
code.

The $(document).ready() construct is actually calling the .ready() method on a
jQuery object we've constructed from the document DOM element. Because this is
a common task, the $() function provides a shortcut for us. When called with no
arguments, the function behaves as though document were passed in. This means
that instead of:
  

 $(document).ready(function() {
       // Our code here...
    });

we can write:

   $().ready(function() {
       // Our code here...
    });

In addition, the factory function can take another function as an argument. When
we do this, jQuery performs an implicit call to .ready(), so for the same result we
can write:
  

 $(function() {
       // Our code here...
    });

While these other syntaxes are shorter, the authors prefer the longer version to make
it clearer what the code is doing.