isPrime.js

// Check if given number is prime
const isPrime = number => {

  if (number < 2) return false;
  
  // Divide number by all numbers from 2 to sqrt(number)
  // If divisible, then its not a prime
  let div;
  for (div = 2; div <= Math.sqrt(number); div++)
    if (number % div == 0)
      return false;

  return true;
}