[JavaScript] Invert binary tree

invert.js

// O(n) time & space
function invertBinaryTree(tree) {
  if (tree === null) return;
  swapLeftAndRight(tree);
  invertBinaryTree(tree.left);
  invertBinaryTree(tree.right);
}

function swapLeftAndRight(tree) {
  const left = tree.left;
  tree.left = tree.right;
  tree.right = left;
}

// This is the class of the input binary tree.
function BinaryTree(value) {
  this.value = value;
  this.left = null;
  this.right = null;
}
/*
  tree =  1
        /  \
       2    3
      / \  / \
     4  5 6  7
    / \
   8  9
*/

const tree = new BinaryTree(1);
tree.left = new BinaryTree(2);
tree.right = new BinaryTree(3);
tree.left.left = new BinaryTree(4);
tree.left.right = new BinaryTree(5);
tree.right.left = new BinaryTree(6);
tree.right.right = new BinaryTree(7);
tree.left.left.left = new BinaryTree(8);
tree.left.left.right = new BinaryTree(9);

invertBinaryTree(tree);