SOURCE

var str = 'aaaadddsssssd'
function Statistics(str) {
    var obj = {}
    for (var i = 0; i < str.length; i++) {
        obj.hasOwnProperty(str[i])?obj[str[i]]++:obj[str[i]] = 1
    }
    var strindex = 1, str2 = '', attr2 = []
    for (var attr in obj) {
        if (obj[attr] > strindex) {//如果没并列的 
            strindex = obj[attr]
            str2 = attr
        }
    }
    for (var attr in obj) {//处理并列第一
        if (obj[attr] == strindex ) {
            strindex = obj[attr]
            attr2.push([attr])
        }
    }
   
        return attr2.length == 1? `${str2}出现${strindex}次 `:`有并列${attr2}都出现${strindex}次`
}


console.log(Statistics(str))