jQuery.extend = jQuery.fn.extend = function() {

 // copy reference to target object

 var target = arguments[0] || {}, a = 1, al = arguments.length, deep = false;

 // Handle a deep copy situation

//这个IF条件基本上总是false,只有当target对象是布尔类型时才会是真,也就是这段代码好像没有什么作用

  if ( target.constructor == Boolean ) {   

  deep = target;

  target = arguments[1] || {};

 }

 // extend jQuery itself if only one argument is passed

 if ( al == 1 ) {

  target = this;

  a = 0;

 }

 var prop;

 for ( ; a < al; a++ )

  // Only deal with non-null/undefined values

  if ( (prop = arguments[a]) != null )

   // Extend the base object

   for ( var i in prop ) {

    // Prevent never-ending loop

    这一段代码不知道怎么搞的，说是用来阻止死循环，但我怎么也不明白他是如何阻止的，好像却了他也没什么影响。

    if ( target == prop[i] )

     continue;

    // Recurse if we're merging object values

    if ( deep && typeof prop[i] == 'object' && target[i] )

     jQuery.extend( target[i], prop[i] );

    // Don't bring in undefined values

    else if ( prop[i] != undefined )

     target[i] = prop[i];

   }

 // Return the modified object

 return target;

};