isPalindrome.js

function isPalindrome(str) {

  str = str.replace(/[^a-z0-9]/i, '').toLowerCase();

  if ( str.length === 0) {
    return true;
  }

  let left = 0;
  let right = str.length - 1;

  while ( left < right ) {
    if ( str[left] !== str[right] ) {
      return false;
    }
    left++;
    right--;
  }

  return true;

}

console.log(isPalindrome('kayak'));
console.log(isPalindrome('radar'));
console.log(isPalindrome('letters'));