get_missing_letters.js

var get_missing_letters = function (str) {

    var alphabets = new Array(26);
    var missing = [];

    //initialize data
    for (var j = 0; j < alphabets.length; j++) {
        alphabets[j] = 0;
    }

    //increases the frequency of a character
    for (var i = 0; i < str.length; i++) {
        var character = str.charAt(i).toLowerCase();
        if (isLetter(character)) {
            alphabets[character.charCodeAt(0) - 97] += 1;
        }
    }

    //look for any characters with frequency 0. If there are any,
    //then it is a missing character
    for (var k = 0; k < alphabets.length; k++) {
        if (alphabets[k] === 0) {
            var asciiCode = k + 97;
            missing.push(String.fromCharCode(asciiCode));
        }
    }

    return missing.join("");

};

function isLetter(character) {
    var asciiValue = character.charCodeAt(0);
    if ((asciiValue >= 65 && asciiValue <= 90) || (asciiValue >= 97 && asciiValue <= 122)) {
        return true;
    }
    return false;
}