function num (str){
            var a = {};
            var b = str.split("");
            // key为元素   值为个数
            for (let i = 0; i < b.length; i++) {
                if(a[b[i]]){
                    a[b[i]]++;
                }else{
                    a[b[i]] = 1;
                }
            }
            // 通过比较找出最大的
            var maxLetter = "";
            var levelNum = 0;
            for (const k in a) {
                if(a[k] > levelNum){
                    levelNum = a[k];
                    maxLetter = k;
                }
            }

            console.log(maxLetter + ":" + levelNum);

            return maxLetter;
        }
        var cc = "dafsfsfasfafaqertyyuuioll,mmnnbvvsfsdfqgsafsafgff";
        num(cc);