???<!--????js?????????????????????E??????????????????-->

 ????/*  //this
    1??this??????????????????????û?????????

    2?????????????????????????.?? ??????????????????????û??????window??
    //"use strict"??????????û???this??????undefined??
    3??????????this?????window??
    //"use strict"?????????????undefined???????????????????????û?????????
    4??????????????????????????????????????????????????????this??????????

    5????????????????????this.xxx=xxx;???this????????????????

    6?????call??apply ?????this??????????????this?????call??apply????????ã????????????³??????call??????
   !!!!!
           ????sum.call(obj,????1,????2);
    ???sum.call(100,200),??ô100????obj??????1????200??????2???û???????????undefined??
    7?????call??????????????ö????????????????ö??????????this???????????????????????
          ????£?call???????????????null??undefined????ô????window
          ?????
    8??apply??call????????? ???????????

    ????sum.apply(obj,[????1,????2]);*/



    ???? DOM ??????????
        //???????????????HTML??????????????????????ø??????????????????????DOM???????????????
        //??????????????????????????????????????????????????????µ????????????????