Solution.js

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function (s) {
    let maxLength = 0,
        left = 0,
        right = 0;
    for (let i = 0; i < s.length; i++) {
        let singleCharLength = getPalLenByCenterChar(s, i, i);
        let doubleCharLength = getPalLenByCenterChar(s, i, i + 1);
        let max = Math.max(singleCharLength, doubleCharLength);
        if (max > maxLength) {
            maxLength = max;
            left = i - parseInt((max - 1) / 2);
            right = i + parseInt(max / 2);
        }
    }
    return s.slice(left, right + 1);
};

function getPalLenByCenterChar(s, left, right) {
    // 中间值为两个字符，确保两个字符相等
    if (s[left] != s[right]) {
        return right - left; // 不相等返回为1个字符串
    }
    while (left > 0 && right < s.length - 1) {
        // 先加减再判断
        left--;
        right++;
        if (s[left] != s[right]) {
            return right - left - 1;
        }
    }
    return right - left + 1;
}

console.log(longestPalindrome("cbbd"));