normalSieve.js

const normalSieve = n => {
    return new Promise((resolve, reject) => {
        // Eratosthenes algorithm to find all primes under n
        var array = [],
            upperLimit = Math.sqrt(n),
            output = [];

        // Make an array from 2 to (n - 1)
        for (var i = 0; i < n; i++) {
            array.push(true);
        }

        // Remove multiples of primes starting from 2, 3, 5,...
        for (var i = 2; i <= upperLimit; i++) {
            if (array[i]) {
                for (var j = i * i; j < n; j += i) {
                    array[j] = false;
                }
            }
        }

        // All array[i] set to true are primes
        for (var i = 2; i < n; i++) {
            if (array[i]) {
                output.push(i);
            }
        }

        resolve(output.length);
    });
};

module.exports = normalSieve;