countSubstrings = (s, count = 0) => {
    for (let i = 0; (i < s.length); i++) {/* Time O(N) */
        const [ odd, even ] = [ i, (i + 1) ];
        /* odd-length: single character center */
        count += isPalindromeFromCenter(s, i, odd); /* Time O(N) */
        /* even-length: consecutive characters center */
        count += isPalindromeFromCenter(s, i, even);/* Time O(N) */
    }

    return count;
}
2 Pointer - Expand Around Center
Time O(N^2) | Space O(1)
https://leetcode.com/problems/palindromic-substrings/
@param {string} s
@return {number}