typeof(a);//undefined
typeof(undefined);//undefined
typeof(NaN);//number
typeof(null);//object
var a="123abc";
typeof(+a);//numberr
typeof(!!a);//boolean
typeof(a+"");//string
1=="1";//true
NaN==NaN;//false
NaN==undefined;//false
''11''+11;//1111
1==='1';//false
parseInt('123abc');//123
typeof(typeof(a));//string