PermMissingElemCodility.js

function sumTo(n) {
  return n * (n + 1) / 2;
}
function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    let arraySum = A.reduce(function(acc, el) { return acc + el; }, 0);
    let sum =sumTo(A.length+1)
    return sum - arraySum
}