#jQuery.fn.init()

接收三个参数，selector,context,rootjQuery

默认的$()传的值实际上考虑了很多种情况，下面是返回值

1. false,undefined,null

    return this;

2. node节点

    this.context = this[0] = selector;
    this.length = 1;
    return this;
    

3. “body”

    this.context = document;
    this[0] = document.body;
    this.selector = selector;
    this.length = 1;
    return this;
    

4. 字符串，单独标签<tag></tag>

    document.createElement;
    return jQuery.merge(this,selector)
    

5. 字符串，复杂的HTML代码<span class=“a” >

    jQuery.buildFragment;
    return jQuery.merge(this,selector)
    

6. 字符串，#id

    document.getElementById;
    this.context = document;
    this.selector = selector;
    return this;
    

7. 字符串，选择器

    return jQuery.find(selector);
    

8. 字符串，选择器，context传值为$(…)

    return context.find(selector);
    

9. 字符串，选择器，context传值为非jquery对象

    return this.constructor(context).find(selector);

   7、8、9可以写成 return (context || rootjQuery).find(selector);

10. 函数

    return rootjQuery.ready(selector);

11. jquery对象//深拷贝

    this.selector = selector.selector;
    this.context = selector.context;

12. 其它

    return jQuery.makeArray(selector,this);