直接上代码

let str = 'apple';
let obj = {};
let number = 0;
let activeIndex = '';

[...str].map(key => {
    key in obj ? obj[key]++ : (obj[key] = 1);

    obj[key] > number ? (activeIndex = key) : '';
    number = obj[key];
});

console.log(activeIndex); // p