JavaScript Program to Print All Prime Numbers in an Interval

let a, b, i, j, flag;

a = 2
b = 11
for (i = a; i <= b; i++) {
    if (i == 1 || i == 0)
        continue;

    // flag variable to tell
    // if i is prime or not
    flag = 1;

    for (j = 2; j <= i / 2; ++j) {
        if (i % j == 0) {
            flag = 0;
            break;
        }
    }

    // flag = 1 means i is prime
    // and flag = 0 means i is not prime
    if (flag == 1)
        console.log(i);
}

let a, b, i, j;
a = 5;
b = 27;
if (a <= 2) {
    a = 2;
    if (b >= 2) {
        console.log(a);
        a++;
    }
}

// Make sure that a is odd before
// we begin the loop
if (a % 2 == 0)
    a++;

// Note: We traverse through odd numbers only
for (i = a; i <= b; i = i + 2) {

    // flag variable to tell
    // if i is prime or not
    let flag = 1;

    // Traverse till square root of j
    // (Largest possible value of a prime factor)
    for (j = 2; j * j <= i; ++j) {
        if (i % j == 0) {
            flag = 0;
            break;
        }
    }

    // flag = 1 means i is prime
    // and flag = 0 means i is not prime

    if (flag == 1) {
        if (i == 1) continue;
        else
            console.log(i);
    }
}

function sieveOfEratosthenesOptimized(start, end) {
    const primes = new Array(end + 1).fill(true);
    primes[0] = primes[1] = false;

    for (let i = 2; i * i <= end; i++) {
        if (primes[i]) {
            for (let j = i * i; j <= end; j += i) {
                primes[j] = false;
            }
        }
    }

    for (let i = Math.max(2, start); i <= end; i++) {
        if (primes[i]) console.log(i);
    }
}

// Take input from the user using prompt
const startInterval = 10;
const endInterval = 20;

// Execute the function with user-provided interval
console.log("Prime numbers between", startInterval,
    "and", endInterval,
    "(Sieve of Eratosthenes - Optimized):");
sieveOfEratosthenesOptimized(startInterval, endInterval);