function asteriscIt(n) {
  // convert parameter to a string
  // remove all commas and split into an array
  let arr = n.toString().replace(/,/g, '').split('');
  let result = [];
  for ( let i = 0; i < arr.length; i++) {
    if ( arr[i] % 2 === 0 && arr[i + 1] % 2 === 0) {
      // if arr[i] is even && arr[i + 1] is even
      result.push(arr[i] + '*')
      // result = arr[i] *
    } else {
      result.push(arr[i])
      //result = arr[i]
    }
  }
  return result.join('')
  // convert array back to string
};

asteriscIt(5312708)
5312708 ==> "531270*8"
asteriscIt([1, 2, 4, 8, 16])
[1, 2, 4, 8, 16] ==> "12*4*816"