missing-number-in-array.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 findMissingNumber (input) {
  if (isIntegerArray(input)) {
    // iterate through all numbers
    for (let i = 0; i < input.length; i++) {
      // check difference between next number and current number
      const difference = input[i+1] - input[i]

      //  if greater than 1, current number + 1 is missing number
      if (difference > 1) return input[i] + 1
    }
  } else return 'Please input an array of integers.'
}

console.log(findMissingNumber([1, 2, 3, 4, 5, 6, 7, 9, 10]))