JSFiddle - React, Tailwind, and code Playground

JavaScript

var findError = function(arr){
    arr = arr.sort(function (a, b) {  return a - b;  });
    for(x = 0; x < arr.length; x++){
        if(arr[x] != x + 1){
           return [arr[x], x + 1];
        }
    }
}

var nums = [1,1, 3, 4, 5, 6, 7, 8];
var nums2 = [1,2, 3, 4, 5, 6, 7, 7];
var nums3 = [1,2, 3, 4, 5, 6, 7, 8, 10, 10];
var nums4 = [1,2, 3, 4, 4, 6];
var nums5 = [1,2, 2, 4];
var nums6 = [1,1];

console.log(findError(nums)); // [1, 2]
console.log(findError(nums2)); // [7, 8]
console.log(findError(nums3)); // [10, 9]
console.log(findError(nums4)); // [4, 5]
console.log(findError(nums5)); // [2, 3]
console.log(findError(nums6)); // [1, 2]