Find all primes till certain limit.

primes.js

var primesUsingEratosthenesMethod = function(num) {
    return new Array(num).fill(1).map((key, index) => index + 1).reduce((primesLessThanCur, cur) => {
        if (cur < 2) return primesLessThanCur;
        if (primesLessThanCur.filter(prime => prime * prime <= cur).every(prime => cur % prime != 0)) {
            return primesLessThanCur.concat(cur);
        }
        return primesLessThanCur;
    }, [])
}

var primesUsingFilter = function(num) {
    function isPrime(number) {
        if (number < 2) return false;
        if (number < 4) return true;
        let limit = Math.sqrt(number);
        for (let i=2; i<= limit; i++) {
            if (number %i == 0) return false;
        }
        return true;
    }
    return new Array(num).fill(1).map((key, index) => index + 1).filter(isPrime)
}

let start = Date.now()
console.log(primesUsingEratosthenesMethod(10000).length, Date.now() - start);
//Output 1229 631


start = Date.now()
console.log(primesUsingFilter(10000).length, Date.now() - start);
//Output 1229 6