Finds the number taken from the mixed array of numbers from 1 to N

find-removed-number.js

// Values for test
const N = 100;
const pos = 34;
const baseArr = Array.from({ length: N }, (v, k) => k + 1);
const removedArr = baseArr.filter(item => item !== pos + 1).sort(() => 0.5 - Math.random());
// -----------------------------------------------------------------------------------------

// Main method
const getRemovedNumber = (arr, maxNumber) => {
  const baseSum = (maxNumber * (maxNumber + 1)) / 2;
  
  return baseSum - arr.reduce((memo, val) => memo + val, 0);
};

// Test
console.log(baseArr[pos] === getRemovedNumber(removedArr, N)); // true