String.prototype.maxCounts = function(){ var res = [], maxCh = '', max = 0; for(let i = 0; i < this.length; i++){ var ch = this[i]; if(res[ch] == null){ res[ch] = 1; }else{ res[ch]++; } if(res[ch] > max){ max = res[ch]; maxCh = ch; } } return "出现次数最多的字符为" + maxCh + ",出现次数为" + max; } document.write("abcdefgaddda".maxCounts());