sieve.js

const argv = require('yargs').argv;

let sieve = {};
let timeAtStart;

// like a boss
Object.prototype.commafy = function () {
	return this.toString().replace(/(^|[^\w.])(\d{4,})/g, function($0, $1, $2) {
		return $1 + $2.replace(/\d(?=(?:\d\d\d)+(?!\d))/g, "$&,");
	});
}

const startTimer = (title = "Timer:") => {
    console.log(title);
    timeAtStart = (new Date()).getTime();
}

const stopTimer = () => {
    const delta = (new Date()).getTime() -  timeAtStart;
    timeAtStart = null;
    console.log(`${(delta / 1000).toFixed(2)} second(s) elapsed`)
    console.log('-----------------------------------------------------------------');
    return delta;
}

const isPrime = (n, useSieve = false) => {
    const numToCheck = parseInt(n);
    if (numToCheck <= 1) { 
        return false;
    }
    if (useSieve && typeof sieve[numToCheck] !== 'undefined') {
        return sieve[numToCheck];
    }
    const top = Math.floor(Math.sqrt(numToCheck));
    
    const prime = [...new Array(top)] // make an array of size top
        .map((v, idx) => idx + 2)     // make the array 2..(top + 2)
        .reverse()                    // reverse
        .reduce((prev, cur) => !!(prev & (numToCheck % cur != 0)), true); // use a reducer to determine primacy

    if (useSieve) { // if we're using the sieve, populate it
        sieve[numToCheck] = prime;
        let mult = numToCheck + numToCheck;
        while (mult < sieveMax) {
            sieve[mult] = false; // all multiples of numToCheck are non-prime
            mult += numToCheck;
        }
    }

    return prime;
}


const checkRange = argv.check || 1000000;
const sieveMax = Math.min(checkRange, argv.sieveMax || 10000000);
let primeCount = 0;

startTimer(`Primes between 1 and ${checkRange.commafy()} WITH sieve:`);
for (x of [...new Array(checkRange)].map((x, i) => i + 1)) {
    if (isPrime(x, true)) {
        primeCount++;
    }
}
console.log(`     ${primeCount.commafy()} primes!`);
stopTimer();

primeCount = 0;

startTimer(`Counting primes between 1 and ${checkRange.commafy()} WITHOUT sieve:`);
for (x of [...new Array(checkRange)].map((x, i) => i + 1)) {
    if (isPrime(x, false)) {
        primeCount++;
    }
}
console.log(`     ${primeCount.commafy()} primes!`);
stopTimer();

sieve = {};