// const inputArr = [4,1,3,14,15,18,39,56,89,101,150,165,187] // output 3
// const inputArr = [9,31,38,5,62,44,38,17,19,38,50,74] // output 5
const inputArr = [5,6,8,9,11,14,16,18,20,25] //output 'null'

const smallestPrime = (numArray) => {
  const inputArr = numArray.slice();
  const isPrime = num => {
      for(let i = 2, s = Math.sqrt(num); i <= s; i++)
          if(num % i === 0) return false;
      return num !== 1;
  }
  const isIndexEvenPosition = num => (inputArr.indexOf(num) + 1) % 2 !== 0
  const primeNumArray = numArray.filter(isPrime)

  let smallestNumPosition = 0 // copy input array
  let smallestNum = primeNumArray.sort((a,b) => a-b)[smallestNumPosition]

  while (isIndexEvenPosition(smallestNum)) {
    smallestNumPosition++;
    smallestNum = primeNumArray[smallestNumPosition]

  }

  return smallestNum > 0 ? smallestNum : 'NULL'
}

console.log(smallestPrime(inputArr))