Code:

import {
    TreeNode,
    ArrayToTree
} from '../common.js'

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */
var kthSmallest = function(root, k) {
    const stack = []

    while(root || stack.length) {
        while(root) {
            stack.push(root)
            root = root.left
        }


        root = stack.pop()

        if (--k === 0) {
            return root.val
        }

        root = root.right
    }
};

console.log(kthSmallest(ArrayToTree([5,3,6,2,4,null,null,1]), 3))