Simple trial division in JavaScript

Question

I began programming recently. I am trying to implement a simple trial-division algorithm for finding all primes up to some number (it is much more "primitive" algorithm than the Sieve of Eratosthenes). Can you please find what's wrong with my code?

#range function:
range = function (a,b,c){
 var range1=[]
 for (i=a; i<b; i=i+c){
 range1.push(i);
 }
 return range1;
}
#The algorithm:
n=prompt("n");
var numbers=range(2,n,1);
var primes=[];
for (number in numbers){
 var sublist=range(2,number,1);
 console.log(sublist);
 for (x in sublist){
 if (number%x ===0){
 break;
 }
 primes.push(number); 
 }
}

Thanks in advance!

• Jeff Vanzella: Is this code working? I'm a little confused at the request "Can you please find what's wrong with my code?"

Answer 1

Try using this instead.

// `isPrime` adopted from http://www.javascripter.net/faq/numberisprime.htm
var isPrime = function (n) {
 if (isNaN(n) || !isFinite(n) || n % 1 || n < 2) {
 return false;
 }
 if (n % 2 === 0){
 return (n === 2);
 }
 if (n % 3 === 0){
 return (n === 3);
 }
 for (var i = 5, m = Math.sqrt(n); i <= m; i += 6) {
 if ((n % i === 0) || (n % (i + 2) === 0)){
 return false;
 }
 }
 return true;
}
var getPrimesUntilN = function (n) {
 n = Math.abs(n);
 var primes = (1 < n) ? [2] : [];
 if (isNaN(n) || !isFinite(n)) {
 return primes;
 }
 for (var i = 3; i <= n; i+=2) {
 if (isPrime(i)) {
 primes.push(i);
 }
 }
 return primes;
};

Input:

getPrimesUntilN(50);

Output:

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]