Javascript is not a yot (http://techteam.wordpress.com/2012/07/18/javascript-is-not-a-toy/)

bench.js

var runOptimized = function() {
	"use strict";
	function getPrimes(){
		 var primeCount, primes, obj;
		 primeCount = 0;
		 primes = new Array(100000);
		 obj = {};
		 obj.getPrimeCount = function() { return primeCount; };
		 obj.getPrime = function(i) { return primes[i]; };
		 obj.addPrime = function(p) { primes[primeCount]=p; primeCount+=1; };
		 obj.isPrimeDivisible = function(candidate){
		 var i, currentPrime;
		 for(i=1; i<primeCount; i+=1){
			 currentPrime = primes[i];
			 if(currentPrime*currentPrime>candidate) { return false; }
			 if((candidate % currentPrime)===0) { return true; }
		 }
		 return false;
		 };
		 return obj;
	 }
	function main(){
		 var p, c;
		 console.log("JS 25,000th prime");
		 console.time("primes_count_optimized")
		 p = getPrimes();
		 c = 1;
		 while(p.getPrimeCount()<25000){
			 if(!p.isPrimeDivisible(c)){
			 p.addPrime(c);
			 //console.log("added prime ", c);
			 }
		 	c++;
		 }
		 console.log(p.getPrime(p.getPrimeCount()-1));
		 console.timeEnd("primes_count_optimized")
	}
	main();
}

var runNormal = function() {
	"use strict";
	function getPrimes(){
		 var primeCount, primes, obj;
		 primeCount = 0;
		 primes = new Array(100000);
		 obj = {};
		 obj.getPrimeCount = function() { return primeCount; };
		 obj.getPrime = function(i) { return primes[i]; };
		 obj.addPrime = function(p) { primes[primeCount]=p; primeCount+=1; };
		 obj.isPrimeDivisible = function(candidate){
		 var i, currentPrime;
		 for(i=1; i<primeCount; i+=1){
			 currentPrime = primes[i];
			 if((candidate % currentPrime)===0) { return true; }
		 }
		 return false;
		 };
		 return obj;
	}
	function main(){
	 var p, c;
	 console.time("primes_count")
	 console.log("JS 25,000th prime");
	 p = getPrimes();
	 c = 1;
	 while(p.getPrimeCount()<25000){
	 	if(!p.isPrimeDivisible(c)){
	 		p.addPrime(c);
	 		//console.log("added prime ", c);
	 	}
	 	c++;
	 }
	 console.log(p.getPrime(p.getPrimeCount()-1));
	 console.timeEnd("primes_count")
	 }
	main();
}

console.log("running normal JS Code");
runNormal();
console.log("running optimized JS Code");
runOptimized();

js_normal.js

"use strict";
function getPrimes(){
 var primeCount, primes, obj;
 primeCount = 0;
 primes = new Array(100000);
 obj = {};
 obj.getPrimeCount = function() { return primeCount; };
 obj.getPrime = function(i) { return primes[i]; };
 obj.addPrime = function(p) { primes[primeCount]=p; primeCount+=1; };
 obj.isPrimeDivisible = function(candidate){
 var i, currentPrime;
 for(i=1; i<primeCount; i+=1){
 currentPrime = primes[i];
 if((candidate % currentPrime)===0) { return true; }
 }
 return false;
 };
 return obj;
 }
function main(){
 var p, c;
 console.time("primes_count")
 console.log("JS 25,000th prime");
 p = getPrimes();
 c = 1;
 while(p.getPrimeCount()<25000){
 if(!p.isPrimeDivisible(c)){
 p.addPrime(c);
 //console.log("added prime ", c);
 }
 c++;
 }
 console.log(p.getPrime(p.getPrimeCount()-1));
 console.timeEnd("primes_count")
 }
main();

js_optimized.js

"use strict";
function getPrimes(){
 var primeCount, primes, obj;
 primeCount = 0;
 primes = new Array(100000);
 obj = {};
 obj.getPrimeCount = function() { return primeCount; };
 obj.getPrime = function(i) { return primes[i]; };
 obj.addPrime = function(p) { primes[primeCount]=p; primeCount+=1; };
 obj.isPrimeDivisible = function(candidate){
 var i, currentPrime;
 for(i=1; i<primeCount; i+=1){
 currentPrime = primes[i];
 if(currentPrime*currentPrime>candidate) { return false; }
 if((candidate % currentPrime)===0) { return true; }
 }
 return false;
 };
 return obj;
 }
function main(){
 var p, c;
 console.log("JS 25,000th prime");
 console.time("primes_count_optimized")
 p = getPrimes();
 c = 1;
 while(p.getPrimeCount()<25000){
 if(!p.isPrimeDivisible(c)){
 p.addPrime(c);
 //console.log("added prime ", c);
 }
 c++;
 }
 console.log(p.getPrime(p.getPrimeCount()-1));
 console.timeEnd("primes_count_optimized")
 }
main();

To test using node:
curl -s https://gist.github.com/fidelisrafael/8971907/raw/7eb1f390687525073c8ad61ad1c99b4d501bc7b4/bench.js > js-bench.js && node js-bench.js