backwardsPrime(2, 100) // => [13, 17, 31, 37, 71, 73, 79, 97] 
backwardsPrime(9900, 10000) // => [9923, 9931, 9941, 9967]

Number.prototype.isPrime = function(){ 
 var maxFactor = Math.floor(Math.sqrt(this)); 
 for(var i=2;i<=maxFactor;i++){ 
 if(this % i === 0){ 
 return false; 
 } 
 } 
 return true; 
};

Number.prototype.palindrome = function(){ 
 return (this + "").split("").reverse().join("") - 0; 
};

function backwardsPrime(start, stop){ 
 var result = []; 
 for(var i=start;i<=stop;i++){ 
 if(i.isPrime() && i.palindrome().isPrime() && i !== i.palindrome()){ 
 result.push(i); 
 } 
 } 
 return result; 
}