Sieve of Eratosthenes

Eratosthenes.js

/**
 * This function implements the Sieve of Eratosthenes.
 * https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
 *
 * @param  {number} lastNumber The upper bound of the number to check
 * @return {number[]}            An array of prime numbers less than lastNumber
 */
function sieve(lastNumber = 1000) {
  // Create an array and fill it with true
  // Important! the array's length is one more than lastNumber so that
  // the array index matches the value of the number we are checking.
  const isPrimeArray = new Array(lastNumber + 1).fill(true);
  // We know that 0 and 1 are not prime, so set it here.
  isPrimeArray[0] = false;
  isPrimeArray[1] = false;

  // The upper bound of what we need to check is the square root of lastNumber
  const squrtLast = Math.floor(lastNumber ** 0.5);
  for (var i = 0; i <= squrtLast; i++) {
    // Only go through this loop is we don't yet know if i is prime
    if (isPrimeArray[i]) {
      for (var j = i ** 2; j < isPrimeArray.length; j += i) {
        // All multiples of i are not prime
        isPrimeArray[j] = false;
      }
    }
  }

  // We need to convert the array from true/false values to the actual numbers
  // that are prime. Do this here
  return isPrimeArray
    // first: convert true values to their index and false values to undefined
    .map((isPrime, index) => isPrime ? index : undefined)
    // filter out all undefined values.
    // What remains are the prime numbers
    .filter(index => index)
    // Reduce the array to find the sum of all the prime numbers
    .reduce((sum, val) => sum += val, 0);
}

const num = 1000;
const sum = sieve(num);
console.log(`The sum of the prime numbers from 0 to ${num} is ${sum}`);