In jQuery, the fn property is just an alias to the prototype property.(source code:jQuery.fn = jQuery.prototype = {...}) The jQuery.fn.extend() method extends the jQuery prototype ($.fn) object to provide new methods that can be chained to the jQuery() function. The jQuery.extend() method Merge the contents of two or more objects together into the first object. (source code:jQuery.extend = jQuery.fn.extend = function() { = arguments[0] || {}, i = 1, length = arguments.length,...})