In the code, the function maxDepth takes two arguments: a root object and an integer. The functionmaxDepth calculates the maximum depth of the two subtrees rooted at the object passed as the argument, and then returns the value of 1 plus the number of nodes in the deepest subtree.
function maxDepth(root) {
if (!root) return 0;
return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
}