function SimpleSymbols(str) { 
    var letter =['a','b','c','d','e','f','g','h','i','j',
    'k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'];
    var newstr = "";
    for (var i = 0; i<str.length; i++){
        if (str.charAt(i).toLowerCase() in letter){
            newstr += "M";
        }
        else{
            newstr += "X";
        }
    }
return newstr; 
}
If str is "Argument goes here" it returns XXXXXXXXX. WHy doesn't it return MMMMMMMMMM?