JavaScript - Find the missing letter

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

Solution Basic approach

function fearNotLetter(str) {
    for (var i = 0; i < str.length; i++) {
        /* code of current character */
        var code = str.charCodeAt(i);

        /* if code of current character is not equal to first character + no of iteration
            hence character has been escaped */
        if (code !== str.charCodeAt(0) + i) {
            /* if current character has escaped one character find previous char and return */
            return String.fromCharCode(code - 1);
    return undefined;



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