Find the missing letter in sequence
In general, you don't need to split the string and then process the array. Simply you must start from the first letter and store its charcode and then process the next later and check with the incremented charcode. If there is a difference than just break.

function f(str) {
    if (str &amp;&amp; 0 &lt; str.length) {
        var desCharCode = str.charCodeAt(0);
        for (var i = 1; i &lt; str.length; ++i) {
            ++desCharCode;
            if (str.charCodeAt(i) != desCharCode) {
                return String.fromCharCode(desCharCode);
            }
        }
    }

    return undefined;
}