Find the missing number in an array

findMissingNumber.js

function findMissingNumber(arr)
{
  const n = arr.length + 1;
  let num = ((n + 1) * n) / 2;

  arr.forEach(val => {
    num = num - val;
  });

  return num;
}

// Numbers from 1 to 100 withouth 51
let test = [...Array(100).keys()].map(x => x + 1).filter(x => x != 51);

findMissingNumber(test); //returns 51