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              // number to be evaluated if prime
var theNumber = 99;
// obtain true|false from isPrime method
var primeResult = isPrime(theNumber);
// write the results 
document.write("<hr />Is " + theNumber + " a prime number: " + primeResult);

function isPrime(n) {
  // single digit primes
  var s = [2,3,5,7];
  // last digit of primes > 10
  var ld = [1,3,7,9];
  // will potentially contain divisibles
  var divs = []; 

  // if 1 is entered, not prime
  if (n==1) {return false;}
  // determine if a single digit prime
  if (s.indexOf(n) > -1) {return true;}
  // if the last digit is potentially prime...
  if (ld.indexOf(n%10) > -1) {
     // determine divisibles
     divs = getDivisibles(n);
  } else {
    // here because number ended with 0,2,4,5,6,or 8
    return false;
  // if no divisbles, it is a prime number
  return (divs.length == 0);
}// isPrime

function getDivisibles(n) {
  // start with an index of 2
  var i = 2;
  // initialize an empty array
  var divs = [];
  // while the index*index is less than the number...
  while (i*i<=n) {
      // if index evenly divides into number...
      if (n%i==0) {
        // determine what index divides into number
        var upper = n/i;
        // add index
        // if index == upper, don't add to array twice
        if (upper>i) {divs.push(upper);}
  }// while
  // sort the array in ascending order
  divs.sort(function(x,y){return x-y;});
  return divs;
}// getDivisibles
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