function missing(arr) {
const max = Math.max(...arr);
return arr.reduce((missing, current, i, s) => {
if(current !== 1 && current !== max) {
let [a, b] = [[], []]
if(!s.includes(current - 1)) a = current - 1;
if(!s.includes(current + 1)) b = current + 1;
return [...missing, ...[].concat(a, b)]
}
return missing;
}, []);
}