list_missing_letters.js

function listMissingLetters(str) {
    let alphabets = "abcdefghijklmnopqrstuvwxyz";
    let missingChars = '';

    for(let i = 0; i < alphabets.length; i++){
        
        if(!str.includes(alphabets[i])){
            missingChars += alphabets[i];
        }
    }
    return missingChars;
}

console.log(listMissingLetters('some letters are missing'));
console.log(listMissingLetters('i can write from a to z: abcdefghijklmnopqrstuvwxyz'));