var str = "aaddaabbcdddefg"

             console.log(str.charAt());//b   没有返回空不是null

             console.log(str.indexOf('p'));//1  没有返回-1

             var obj  = {};

             for(var i=;i<str.length;i++){

                 var v = str.charAt(i);

                 if(obj[v] && obj[v].value == v){

                     obj[v].count  ++ ;

                 }else{

                     obj[v] = {};

                     obj[v].count = ;

                     obj[v].value = v;

                 }

             }

             console.log(obj);//true

             //obj =  {a:object,b:object,c:object}

             for(key in obj){

                 document.write(obj[key].value +'='+obj[key].count+'&nbsp;'); // a=4  b=3  c=4  d=2  f=1  g=1  h=1

             }