소수 찾기 도움말

https://school.programmers.co.kr/learn/courses/30/lessons/12921

function solution(n) {
    let arr = [...Array(n+1).keys()]
    for (let i=2; i<=Math.sqrt(n); i++) {
        let j=2;
        if (arr[i]!==false) {
            while (i*j<=n) {
                arr[i*j]=false
                j+=1
            }
        }
    }
    return arr.reduce((cnt, element) => cnt - (element === false), n-1);
}