jQuery全局命名空间中定义的唯一两个变量 jQuery $
$(document).ready(fn);//== 在DOMContentLoaded时触发，浏览器不支持时，会在load时触发 $(fn) jQuery.noConflict();//还原$()为初始值
jQuery对象返回的类数组对象,拥有length属性，包含selector(选择字符串),context(对象上下文),jquery(jQuery版本号，用于区分jquery对象)
常见API
.each(): Iterate over a jQuery object, executing a function for each matched element.
在回调函数中调用return false来终止循环
.map() : Pass each element in the current matched set through a function, producing a new jQuery object containing the return values.
.index() : Search for a given element from among the matched elements.
If the element is not found, .index() will return -1.
is() : Check the current matched the set of elements against a selector, element, or jQuery object and return true if at least one of these elements matches the given arguments.