查找字符串中出现最多的字符和个数

var str = 'safsafsagadsfshjsadfhsajdhgsaaaaaaaaaaaafsafasdvgdasfsajkodhsaodsagdusiagdusaidhsajdhsajdhsajdhsaohdiusahduisahdusadhudhufsaaaaaagfdghdfhdfhdfhdfgjgfjfgjfddfhfghhdfhdfhdfhdsfgfdghdfhdghfgfdghdfgdfgdfgdsdfgsdfgsgdfgsdgsdfdsfg';
    var obj = {};
    var num = 0; 
    var value = '';
    function test( str ){
            for(var i=0; i<str.length; i++){
            if( !obj[str[i]] ){
                obj[str[i]] = [];
            }
            obj[str[i]].push( str[i] );
        }

        for(var attr in obj){
            if( num < obj[attr].length ){
                num = obj[attr].length;
                value = obj[attr][0];
            }
        }
        return value + ':' +num;
    }
    console.log( test(str) );