MRUAsList.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;
}

// Memoize is a Proxy to the actual function
const memoize = (fn, mru=10) => new Proxy(fn,

  // Handler with traps
  {
    // Instead of object, let's use a list which contains
    // [ {args:2, result:true}, {args:3, result:true}, {args:4, result:false} ]
    cache: [],  
    max: mru,   // Our cache limit

    // Our "magic" property
    get(object, property, receiver) {
      if (property == "cache")
        return this.cache;
      
      return Reflect.get(object, property, receiver);
    },

    // Intercept call to the wrapped function
    apply(target, thisArg, args) {

      // Scan the list to see if we have args cached
      let exists = this.cache.find(element => {
        element.args == args.toString();
      });

      // If exists, return the result
      if (exists) {
        result = exists.result;
      } else {
        // New result, store it at the end
        result = Reflect.apply(target, thisArg, args);
        this.cache.push({ args: args.toString(), result: result });
      }

      // Have we exceeded the limit? Drop the first element
      if (this.cache.length > this.max)
        this.cache.shift();
      
      return result;
    }
  }
);

// Create the proxy and go thru max numbers to test for isPrime
let smartPrime = memoize(isPrime);
let max = 1000000;
let primeCount = 0;
for (let number = 2; number <= max; number++)
  if (smartPrime(number)) {
    primeCount++;
  }

console.log(`Number of primes between 0 and ${max}: ${primeCount}`);

// Let's test 2 again
console.log("2 is prime", smartPrime(2));

console.log(`Cache size: ${smartPrime.cache.length}`);
console.log('Cache:', smartPrime.cache);

/* Output
Number of primes between 0 and 1000000: 78498
2 is prime true
Cache size: 10
Cache: [
  { args: '999992', result: false },
  { args: '999993', result: false },
  { args: '999994', result: false },
  { args: '999995', result: false },
  { args: '999996', result: false },
  { args: '999997', result: false },
  { args: '999998', result: false },
  { args: '999999', result: false },
  { args: '1000000', result: false },
  { args: '2', result: true }
]
*/