Find missing letter in list of alphabets

Question

I am trying to solve the following issue:

Find the missing letter in the passed letter range and return it. If all letters are present in the range, return undefine

Answer 1

I think this is the simplest code to do this:

function skippedLetter(str) {
    for (var i = 0; i < str.length - 1; i++) {
        if (str.charCodeAt(i + 1) - str.charCodeAt(i) != 1) {
            return String.fromCharCode(str.charCodeAt(i) + 1);
        }
    }
}

alert(skippedLetter('abce'));

This version will reject illegal input, accept both upper and lower case, check that there is only 1 hole in the range, and that there is exactly 1 character missing.

function skippedLetter(str) {
    if (!str.match(/^[a-zA-Z]+$/)) return;
    var letter = "", offset = str.charCodeAt(0);
    for (var i = 1; i < str.length; i++) {
        var diff = str.charCodeAt(i) - i - offset;
        if (diff == 1) letter += String.fromCharCode(i + offset++)
        else if (diff) return;
    }
    if (letter.length == 1) return letter;
}

alert(skippedLetter('123567'));		// illegal characters
alert(skippedLetter(''));		// empty string
alert(skippedLetter('a'));		// too short
alert(skippedLetter('bc'));		// nothing missing
alert(skippedLetter('df'));		// skipped letter = e
alert(skippedLetter('GHIKLM'));		// skipped letter = J
alert(skippedLetter('nOpRsT'));		// cases mixed
alert(skippedLetter('nopxyz'));		// too many characters missing
alert(skippedLetter('abcefgijk'));	// character missing more than once
alert(skippedLetter('abcefgfe'));	// out of order