euler4.js

function getDigitAt(position, value) {
  return Math.floor((value % 10 ** position) / 10 ** (position - 1));
}

function getLength(value) {
  return Math.floor(Math.log10(value)) + 1;
}

function isPalindrome(value) {
  const digits = getLength(value);
  const halfDigits = Math.floor(digits / 2);
  for (let i = 0; i < halfDigits; i++) {
    if (getDigitAt(i + 1, value) !== getDigitAt(digits - i, value)) {
      return false;
    }
  }
  return true;
}

function getLargestPalindrome(digits1, digits2) {
  const max1 = 10 ** digits1 - 1;
  const max2 = 10 ** digits2 - 1;
  const min1 = 10 ** (digits1 - 1) - 1;
  const min2 = 10 ** (digits2 - 1) - 1;
  let largest = 0;
  for (let i = max1; i > min1; i--) {
    for (let j = max2; j > min2; j--) {
      const sum = i * j;
      if (sum > largest && isPalindrome(sum)) {
        largest = sum;
      }
    }
  }
  return largest;
}


getLargestPalindrome(3, 3);