function countMostFrequentLetters(string) {

const letterCounts = {};

for (const char of string) {

// check if char is alphabet

if (char.toLowerCase() !== char.

) {

const lowerChar = char.toLowerCase();

letterCounts[lowerChar] += letterCounts[lowerChar] ?

: 0;

}

}

const maxCount = Math.max(...Object.values(letterCounts));

// print most popular letters

const mostPopularLetters = [];

for (const [key, value] of Object.

(letterCounts)) {

if (value === maxCount) {

mostPopularLetters.push(

);

}

}

return mostPopularLetters;

}

console.log(countMostFrequentLetters('mango and oranges')); // ['a', 'n']

console.log(countMostFrequentLetters('mississippi')); // ['i', 's']