find-duplicate-numbers.js

// validate input array elements
function isIntegerArray (input) {
  // if an array
  if (Array.isArray(input)) {
    // if empty array
    if (input.length < 1) return false

    // iterate through each element and return false if one of them happens to be not an integer
    for (let i = 0; i < input.length; i++) {
      if (Number.isInteger(input[i])) continue
      else return false
    }
    return true
  // if not an array
  } else return false
}

function findDuplicateNumbers (input) {
  let duplicates = []

  if (isIntegerArray(input)) {
    // for integers being compared
    for (let i = 0; i < input.length; i++) {
      // for integers being compared with
      for (let j = 0; j < input.length; j++) {
        // if not comparing to self
        if (!(i === j)) {
          // AND if found similar with each other,
          // AND if isn't already in 'duplicates' array
          // push into 'duplicates' array
          if (
            (input[i] === input[j]) &&
            !duplicates.includes(input[i])
          ) duplicates.push(input[i])
        }
      }
    }

    return duplicates
  } else return 'Please input an array of integers.'
}

console.log(findDuplicateNumbers([1, 68, 3, 4, 5, 68, 9, 1, 1]))