示例：以aabccdeefff为例

var arr1 = "aabccdeefff"; 
        var has = [];
        arr1 = arr1.split('');
        for(var i = 0; i < arr1.length; i++) {
            var num = arr1[i];
            if(has.indexOf(arr1[i]) == -1) {
                has.push(arr1[i]);
                
            }
            if(has[num]) { //如果存在
                has[num]++;
            } else { //如果没有
                has[num] = 1;
            }
        }
        console.log(has)

封装成可传参的函数

var arr1 = "aabccdeefff"; //传入参数
function getArr(arr1){
         var has = [];
        arr1 = arr1.split('');
        for(var i = 0; i < arr1.length; i++) {
            var num = arr1[i];
            if(has.indexOf(arr1[i]) == -1) {
                has.push(arr1[i]);
                
            }
            if(has[num]) { //如果存在
                has[num]++;
            } else { //如果没有
                has[num] = 1;
            }
        }
return has;
}