SOURCE

/**
 * @param {string} s
 * @param {string} t
 * @return {character}
 */
var findTheDifference = function(s, t) {
  var m=0;
    for(var i=0;i<t.length;i++){
        m=m^s.charCodeAt(i)^t.charCodeAt(i);
    }
  return String.fromCharCode(m);
};
console.log(findTheDifference('abcd','abcde'))