primes count

primes.js

/**
 * @param {number} n
 * @return {number}
 */
var countPrimes = function(n) {
    if (n < 3) return 0;
    var primes = new Array(n);
    primes.fill(true, 0, n);
    for (var i = 4; i < n; i +=2) primes[i] = false;
    var j = 3;
    var sqrt = Math.sqrt(n);
    while (j < sqrt) {
        var t = j+j;
        while (t < n) {
            primes[t] = false;
            t += j;
        }
        j++;
        while (j < n && !primes[j]) j++;
    }
    
    /*
    var c = 1;
    for (var i = 3; i < n; i += 2) c += (primes[i] ? 1 : 0);
    return c
    */
    return primes.reduce((acc,e) => acc + (e ? 1: 0), -2);
};

var x = new Date();
console.log("result", countPrimes(10000000), "computed in", new Date() - x, "ms")