Get Lowest/Smallest Common Multiple From a Sequential Range of Numbers

lcm.js

/* get the lowest common multiple from a sequential range of numbers
expect arr = [a, b], where `a` and `b` are the (inclusive) edge range of natural numbers
*/
function smallestCommons(arr) {
  let min = Math.min(...arr), max = Math.max(...arr),
  numList = Array.from(Array(max - min +1).keys(), (val) => val + min),
  factorMap = {};
  
  numList.forEach((val) => {  // generate the prime factors count mapping
    for (let [key, value] of Object.entries(primeFactor(val))) {
      if (!factorMap.hasOwnProperty(key)) factorMap[key] = value;
      if (factorMap[key] < value) factorMap[key] = value;
    }
  })

  return [...Object.entries(factorMap)].map(
    (val) => { return Math.pow(parseInt(val[0]), val[1])}
  ).reduce((acc, val) => acc * val);
}

/* get the prime factors of any given natural number above 1, using Sieve of Eratosthenes */
function primeFactor(num) {  // prime factorization
  let factors = [];

  // handle even prime number
  while (num % 2 === 0) {
    factors.push(2);
    num /= 2;
  }

  if (num >= 3) {  // starting from the smallest odd-number prime
    // handle the odd prime numbers
    Array.from(
      Array(Math.ceil(Math.sqrt(num)) - 3 + 1).keys(),
      (val) => val * 2 + 3
    ).forEach((val) => {
      while (num % val == 0) {
        factors.push(val);
        num /= val;
      }
    });
    
    // leftover number is prime
    if (num > 2) factors.push(num);
  }
  
  // create a prime factors mapping `{prime_number[1]: index[1], ...prime_number[n]: index[n]}`
  // where prime_num ^ index = factor of number
  let primeFactorMap = {};
  factors.forEach((val) => {
    if (!primeFactorMap.hasOwnProperty(val.toString())) {
      primeFactorMap[val.toString()] = 0
    }
    primeFactorMap[val.toString()] += 1;
  })

  return primeFactorMap;
}

console.log(smallestCommons([23, 18]));  // Output: 6056820