jQuery function and document ready the same

Question

To have a working datepicker on a field, I have to put this script inside my element

$( function() {
  $( "#date_datepicker" ).datepicker( { dateFormat: "yy-mm-dd" } );
});

Removing the $( function() { makes the datepicker not work.

So does it mean that the $( function() { is the same as $(document).ready?

I'm trying to optimize my javascript codes so knowing this might help.