Prime numbers generator with simple caching

prime.js

/**
 * Prime numbers generator
 * @constructor
 */
function Prime() {
  this.primes = [1];
}

/**
 * Test for primality
 * @method isPrime
 * @param {Number} num
 */
Prime.prototype.isPrime = function(num) {
  if (num !== 2) {
    if (num % 2 === 0) {
      return false;
    }
    else {
      for (x=3; x<=Math.sqrt(num); x+=2) {
        if (num % x === 0) {
          return false;
        }
      }
    }
  }
  
  return true;
}

/**
 * Get next prime number
 * @method getNext
 */
Prime.prototype.getNext = function() {
  var current = this.primes[this.primes.length-1];
  
  current++;
  while (!this.isPrime(current)) current++;
  this.primes.push(current);
  
  return current;
}

/**
 * Get some prime numbers
 * @method getSome
 * @param {Number} qty Quantity
 * @param {Number} [from] Start position
 */
Prime.prototype.getSome = function(qty, from) {
  from = from || this.primes.length;
  
  for (var i=0; i<qty; i++) {
    if (!this.primes[from+i]) {
      this.getNext();
    }
  }
  
  return this.primes.slice(from, from+qty);
}