数组[[1, ‘a’], [2, ‘b’], [3, ‘c’], [4, ‘a’]]，输出出现次数最多的字母的数量和 5

    1. // 利用对象key的唯一性，forEach，取key和value，对象计出现次数count，总和num
    2. function testFn (arr) {
    3.   let obj = {},
    4.       max = 0,
    5.       sum = 0;
    6.   arr.forEach(item => {
    7.     let key = item[1],
    8.         value = item[0];
    9.     if (obj[key]) {
    10.       obj[key].count++;
    11.       obj[key].num += value;
    12.       if (obj[key].count > max) {
    13.         max = obj[key].count;
    14.         sum = obj[key].num;
    15.       }
    16.     } else {
    17.       obj[key] = {count: 1, num: value}
    18.     }
    19.   });
    20.   return sum;
    21. }