showPrimeNunber created by Mrshinezee - https://repl.it/@Mrshinezee/showPrimeNunber

showPrimeNunber.js

let n = 20;
function isPrime(n) {
  for (let i = 2; i < n; i++) {
    if ( n % i == 0) return false;
  }
  return true;
}
isPrime(n);

function showPrimes(n) {

  for (let i = 2; i < n; i++) {
    if (!isPrime(i)) continue;

    alert(i);  // a prime
  }
}
showPrimes(n);