function isPalindrome(str) {

  let left = 0;

  let right = str.length - 1;

  let leftChar;

  let rightChar;

​

  while (left < right) {

    leftChar = str.charAt(left);

    rightChar = str.charAt(right);

​

    if (leftChar == rightChar) {

      left++;

      right--;

    } else {

      return false;

    }

  }

​

  return true;

}

​

console.log(isPalindrome('racecar'));