A function to get all prime numbers below a specified value.

get-primes.js

function sieve(n){
    let arr = new Array(n);
    for(let i = 0; i< arr.length; i++){
        arr[i] = true;
    }

    arr[0] = null;
    arr[1] = null;

    function setToFalseEvery(n){
        return arr.forEach((item, i, arr) => {
            if(i < 2 || i % n !== 0){
                return arr[i] = item;
            }

            return arr[i] = false;
        });
    }

    function findFirstTrueItemIndex(){
        return arr.findIndex(item => item === true);
    }

    const squareRootOfN = Math.round(Math.sqrt(n));
    const arr2 = [];
    let i2;
    for(let i=2; i <= squareRootOfN; i++){
        i2 = findFirstTrueItemIndex();
        arr2.push(i2);
        setToFalseEvery(i2);
    }

    arr = arr.reduce((acc, item, i) => {
        if(item === true){
            acc.push(i);
        }
        return acc;
    }, []);

    return [...arr2, ...arr];
}

console.log(sieve(999999).toString());