isPrimefromMDN.js

// Using a function in arr.filter(return fun) to check true or false

function Prime(element, index, array) {
  var start = 2;
  while (start <= Math.sqrt(element)) {
    if (element % start++ < 1) {
      return false;
    }
  }
  return element > 1;
}

[5, 6, 4, 55, 53, 7].filter(e => {
  return Prime(e);
});

// [ 5, 53, 7 ]

sieveOfEratosthenesCodecademy.js

/*-----------------------------------------------------------------------------
whether it's a prime
the references are from 
  UdemyCourse: LearningAlgorithmsInJavascriptFromScratch,
  www.codecademy.com/en/forum_questions/51c858349c4e9dd24201011d
  MDN Array.prototye.find()
-----------------------------------------------------------------------------*/

function isPrime(n) {
  var result = [];
  for (var i = 2; i <= n; i++) {
    var guessPrime = true;
    for (var j = 2; j < Math.sqrt(n); j++) {
      if (i % j === 0 && j !== i) {
        guessPrime = false;
      }
    }
    if (guessPrime) {
      result.push(i);
    }
  }

  return result;
}

sieveOfEratostheneUdemy.js

function sieveOfEratosthene(n) {
  var originArr = [false, false];
  for (let i = 2; i <= n; i++) {
    originArr.push(true);
  }

  for (var i = 2; i < Math.sqrt(n); i++) {
    for (var j = 2; j * i <= n; j++) {
      originArr[i * j] = false;
    }
  }

  var arePrimes = [];
  var a = originArr.forEach((n, i) => {
    if (n) {
      arePrimes.push(i);
    }
  });

  return arePrimes;
}