PrimeWithCache.js

// We store previous results here
const cache = {};

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

  // Check cache if we have previous result
  if (number in cache) {
    console.log(`Found ${number} in cache: ${cache[number]}`);
    return cache[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) {
      console.log(`Looped ${div} times`);
      cache[number] = false;
      return false;
    }
  console.log(`Looped ${div} times`);
  cache[number] = true;
  return true;
}

// This is a known prime
let number = 1299827;
console.log(`${number} is prime: ${isPrime(number)}`);
console.log(`${number} is prime: ${isPrime(number)}`);

/* Output
Looped 1141 times
1299827 is prime: true
Found 1299827 in cache: true
1299827 is prime: true
*/