How can I decide that a number is prime number?

Question

I have to ask for a number with "var n=prompt" and after that I have to determine that that number is prim or not.
I have been learning cycles in Javascript and I didn't find any answer for that.

Answer 1

the most basic way is to iterate from one to that number continually dividing by that number.if there are no divisors,its a prime.

var is_prime=true
for(var i=2;i<num;i++)
if(num%i===0) is_prime=false;
console.log(is_prime);

this is very inefficient for large primes like 224346557567

Answer 2

Thank you for your help!
:)