jQuery对象的入栈：实质就是返回一个新的jQuery对象，新的jQuery对象的prevObject属性保存着前一个(当前)jQuery对象。

var p = $('p');//选中三个空的p
var p1 = $();
console.log(p);
console.log(p1);

<script>
	window.onload = function(){
	    $('p').pushStack($('span')).css('background-color','red').prevObject.css('background-color','yellow');
    }
 </script>

或者将prevObject替换为end()：

end: function() {
	return this.prevObject || this.constructor(null);
	},

$('p').pushStack($('span')).css('background-color','red').end().css('background-color','yellow');

pushStack: function( elems ) {

		// Build a new jQuery matched element set
		var ret = jQuery.merge( this.constructor(), elems );

		// Add the old object onto the stack (as a reference)
		ret.prevObject = this;
		ret.context = this.context;

		// Return the newly-formed element set
		return ret;
	},

constructor: jQuery,this.constructor()创建一个空的jQuery对象，

工具方法merge():第二个参数必须要有数值属性。

merge: function( first, second ) {
		var l = second.length,
			i = first.length,
			j = 0;

		if ( typeof l === "number" ) {
			for ( ; j < l; j++ ) {
				first[ i++ ] = second[ j ];
			}
		} else {
			while ( second[j] !== undefined ) {
				first[ i++ ] = second[ j++ ];
			}
		}

		first.length = i;

		return first;
	},

<script>
	window.onload = function(){
		var test = document.getElementById('demo');
	    $('p').pushStack(test).css('background-color','red').end().css('background-color','yellow');
    }
   </script>
	

这是因为test没有length属性，如果将test变成数组形式[test]，就可以将span元素的背景显示红色

$('p').pushStack([test]).css('background-color','red').end().css('background-color','yellow');