Is it a credit card function for JS class

isItCreditCard.js

Number.prototype.isCreditCard = (
	function() {
		var testCC = this.toString();
		var sum =0; mul = 1; l = testCC.length;
		
		if (l < 19) {
			for (i = 0; i < l; i++)
			{
		          var digit = testCC.substring(l-i-1,l-i);
		          var tproduct = parseInt(digit ,10)*mul;
		          if (tproduct >= 10)
		               sum += (tproduct % 10) + 1;
		          else
		               sum += tproduct;
		          if (mul == 1)
		               mul++;
		          else
		               mul--;
			}
			
			if ((sum % 10) == 0) {
		          return true;
			}
		 
		} 
		
		return false;
	}	
);

(5555555555554444).isCreditCard();