[Invert Binary Tree] Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off. https://leetcode.com/problems/invert-binary-tree/#.VXqvIdRFNy8.twitter

invertTree.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
    if (root) {
        var left  = root.left  ? root.left  : null,
            right = root.right ? root.right : null;
        root.left  = invertTree(right);
        root.right = invertTree(left);
    }
    return root;
};

You know that's an algorithm for reversing (creating a mirror of a tree), right?