SOURCE

function find1(arr) {
    let result = Array.from({ length: 10 }, (v, k) => k + 1);
    let tempSet = new Set(arr);
    return result.filter(item => !tempSet.has(item));
}

const arr1 = [1, 2, 3, 4, 5, 6, 7, 8, 9];
console.log(find1(arr1));
const arr2 = [1, 2, 3, 1, 1, 1, 1, 1];
console.log(find1(arr2));

function find2(arr, n) {
    let sum = arr.reduce((a, b) => a + b);
    let tempSum = n * (1 + n) / 2;
    return tempSum - sum;
}
const arr3 = [1, 2, 3, 4, 5, 6, 7, 8, 9];
console.log(find2(arr3, 10));
const arr4 = [10, 2, 3, 4, 5, 6, 7, 8, 9];
console.log(find2(arr4, 10));

function find3(arr) {
    let sum = 0;
    let tempSum = arr.length + 1;
    for (let i = 0; i < arr.length; i++) {
        sum += arr[i];
        tempSum += i + 1;
    }
    return tempSum - sum;
}
const arr5 = [1, 2, 3, 4, 5, 6, 7, 8, 9];
console.log(find3(arr5, 10));
const arr6 = [10, 2, 3, 4, 5, 6, 7, 8, 9];
console.log(find3(arr6, 10));