Binary tree postorder traversal in JavaScript

Problem

Given the root of a binary tree, return the postorder traversal of its nodes’ values.

Example 1:

Input: root = [1,null,2,3]
Output: [3,2,1]

Solution

/**
 * 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
 * @return {number[]}
 */
var postorderTraversal = function(root) {
    let result = new Array();
    if(root){
        if(root.left == null & root.right == null){
            result.push(root.val);
        }
        else if(root.left != null || root.right != null){
            result = postordertraverse(root,result);
        }
    }
    return result;
};
function postordertraverse(root,arr){
    if(!root){
        return null;
    }
    else{
        postordertraverse(root.left,arr);
        postordertraverse(root.right,arr);
        arr.push(root.val);
    }
    return arr;
}