Go straight to code

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
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