jQuery源码分析-----pushStack

pushStack是jQuery底层方法

pushStack方法虽然在高层不会用到，但很多方法都会调用它

举例：
html结构为ul下5个li

var li = $('li');var s = li.slice(2);var end = s.end();pushStack：// Take an array of elements and push it onto the stack// (returning the new matched element set)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;}slice: function() {    var temp = slice.apply( this, arguments );    return this.pushStack( temp );}

1. 当执行li.slice(2)时，进入slice方法，this指的是li,temp为后3个li,返回this.pushStack,进入pushStack方法，参数为temp，首先将temp封装成jQuery对象并赋值给ret，即待返回的对象，将this赋值给ret的prevObject属性（方便后续调用end方法），返回ret。此时ret的结构包括

{    jQuery:temp    prevObject:li    context:document}

1. 当执行s.end()时：

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

返回s的prevObject，即li