Count duplicate characters in a string

function countLetters(str) {
    let newStr = str.toLowerCase();
    let count = {}
    for (let letter of newStr) {
        if (count[letter]) {
            count[letter] += 1;
        } else {
            count[letter] = 1;
        }
    }

    return count;
}

console.log(countLetters("canada"));
//Prints: {c:1, a:3, n:1, d:1}

console.log(countLetters("javascript"));
//Prints: {j:1, a:2, v:1, s:1, c:1, r:1, i:1, p:1, t:1}