1) Generate Palindromic numbers and 2) In a list of numbers, get multiples of a given number

palindromic-numbers.js

function makePalindromicNumber(number) {
  const regularNumber = number.toString();
  const reversedNumber = number.toString().split("").reverse().join("");

  return Number(`${regularNumber}${reversedNumber}`);
}

function generatePalindromicNumberList(digits = 2) {
  if (digits < 1) {
    throw new Error("'digits' should be equal or greater than '1'.");
  }

  const initialValue = `1${Array(digits - 1)
    .fill("0")
    .join("")}`;

  const finalValue = `9${Array(digits - 1)
    .fill("9")
    .join("")}`;

  const numbers = [];

  for (let number = initialValue; number <= finalValue; number++) {
    numbers.push(makePalindromicNumber(number));
  }

  return numbers;
}

function getMultiplesOfNumberInList(divisor, array) {
  const numbers = array
    .map((number) => (number % divisor === 0 ? number : null))
    .filter(Boolean);

  const amount = numbers.length;

  return {
    amount,
    numbers,
  };
}


// Example usage
const palindromicNumbers = generatePalindromicNumberList(2);
const multiplesOf15 = getMultiplesOfNumberInList(15, palindromicNumbers);
// <- { amount: 3, numbers: [ 5115, 5445, 5775 ] }