Solution.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */

/**
 * Encodes a tree to a single string.
 *
 * @param {TreeNode} root
 * @return {string}
 */
var serialize = function (root) {
    return rserialize(root, '');
};

/**
 * Decodes your encoded data to tree.
 *
 * @param {string} data
 * @return {TreeNode}
 */
var deserialize = function (data) {
    const dataArray = data.split(',');
    return rdeserialize(dataArray);
};

const rserialize = (root, str) => {
    if (root === null) {
        str += '#,';
    } else {
        str += root.val + '' + ',';
        str = rserialize(root.left, str);
        str = rserialize(root.right, str);
    }
    return str;
};

const rdeserialize = dataList => {
    if (dataList[0] === '#') {
        dataList.shift();
        return null;
    }

    const root = new TreeNode(parseInt(dataList[0]));
    dataList.shift();
    root.left = rdeserialize(dataList);
    root.right = rdeserialize(dataList);

    return root;
};

/**
 * Your functions will be called as such:
 * deserialize(serialize(root));
 */