<h1>
  Prime numbers - Sieve of Eratosthenes
</h1>
<h2>
  Written in pure JavaScript
</h2>

Prime numbers with max limit of <label for="console" id="consoleLabel">500</label><br />
<textarea name="console" id="console" cols="80" rows="20" readonly tabindex="2"></textarea>

<br />
<input type="number" name="limit" id="limit" value="500" min="2" tabindex="1" onchange="writePrimes(this.value)" />

body {
  background-color: linen;
}
#consoleLabel,
#limit {
  font-weight: bold;
  color: tomato;
}
#console {
  font-family: "Lucida Sans", "Lucida Sans Regular", "Lucida Grande",
    "Lucida Sans Unicode", Geneva, Verdana, sans-serif;
  background-color: black;
  color: limegreen;
}
#limit {
  height: 40px;
  line-height: 2em;
  padding: 10px;
}

/* Sieve of Eratosthenes
   Author: https://github.com/Amarok24
   Version: 0.2
   Date: 2020-01-07

   License: The Unlicense. For more information, please refer to http://unlicense.org
   (A license with no conditions whatsoever which dedicates works to the public domain. Unlicensed works, modifications, and larger works may be distributed under different terms and without source code.)
*/

"use strict";

function getPrimes(maxLimit) {
  let field = [];
  let primes = [2];
  let currentPrime = 2;
  let currentIndex = 2;

  let stopAfter = Math.floor(Math.sqrt(maxLimit));
  // stopAfter = highest index after which no other primes
  // will be sieved (simplest code optimization)

  if (maxLimit < 2) {
    return [-1];
  } else if (maxLimit === 2) {
    return primes;
  }

  for (let i=0; i<=maxLimit; i++) {
    field.push(i); // field will contain [0, 1, 2, 3, ... maxLimit]
  }


  while (currentPrime <= stopAfter) {
    // with maxLimit of 50 this will run up to currentPrime 7, not beyond

    currentIndex += currentPrime;
    while (currentIndex <= maxLimit) {
      field[currentIndex] = 0; // mark as removed from field
      currentIndex += currentPrime;
    }

    // now let's go to the the very next prime number in field
    currentIndex = currentPrime;
    while (true) {
      currentIndex++;
      if (field[currentIndex] !== 0) {
        break; // break while loop
      }
    }

    currentPrime = currentIndex;
    primes.push(currentPrime);
  }


  // now let's populate the primes array with all remaining numbers from field
  currentIndex++;
  for (let i = currentIndex; i <= maxLimit; i++) {
    if (field[i] !== 0) {
      primes.push(field[i]);
    }
  }

  return primes;
}


function writePrimes(maxLimit) {
  let output = document.getElementById("console");
  let label = document.getElementById("consoleLabel");
  let p = getPrimes(maxLimit);
  let outS = "", i = 0;
  const lastI = p.length-1;

  label.innerText = maxLimit.toString();

  output.innerText = "";
  while (i<p.length) {
    outS += p[i].toString();
    if (i !== lastI) outS += ", ";
    i++;
  }

  output.innerText = outS;
}


writePrimes(500);