jquery实现全选和全不选功能

网上好多代码都是错的

 

<! DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" >
< html  xmlns ="http://www.w3.org/1999/xhtml" >
< head >
< meta  http-equiv ="Content-Type"  content ="text/html; charset=utf-8"   />
< title >无标题文档 </ title >
</ head >
< script  src ="jquery-1.7.1.min.js" ></ script >
< script  type ="text/javascript" >
    $(document).ready( function  () {

        $( " #checkedAll " ).click( function  () {

             // alert("aa");


             if  ($( this ).attr( " checked " )) {  //  全选 

                $( " input[name='checkbox_name'] " ).each( function  () {
                    $( this ).attr( " checked " ,  true );
                });
            }
             else  {  //  取消全选 

                $( " input[name='checkbox_name'] " ).each( function  () {
                    $( this ).attr( " checked " ,  false );
                });
            }
        });

    }); 

</ script > 

< body >
< input  type ="checkbox"  name ="checkbox_name"  id ="checkbox_name_1"   />1 < br  />
< input  type ="checkbox"  name ="checkbox_name"  id ="checkbox_name_2"   />2 < br  />
< input  type ="checkbox"  name ="checkbox_name"  id ="checkbox_name_3"   />3 < br  />
< input  type ="checkbox"  name ="checkbox_name"  id ="checkbox_name_4"   />4 < br  />
< input  type ="checkbox"  name ="checkedAll"  id ="checkedAll" />全选/取消全选
</ body >
</ html >