JS function to determine how many leaves are in a binary tree

leavesBinaryTree.js

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+'use strict';
+
+// JavaScript implementation of a function that finds the number of leaves in a binary tree
+// function's signature is a binary tree as a parameter, returns the number of leaves in the tree
+// relies on the assumption that the input is a binary tree with a root, and made up of nodes with left and right properties on them, as illustrated below
+// function uses preOrder depth traversal method
+
+class Node {
+
+  constructor(value, left = null, right = null) {
+    this.value = value;
+    this.left = left;
+    this.right = right;
+  }
+
+}
+
+class BinaryTree {
+
+  constructor( root = null) {
+    this.root = root;
+  }
+
+}
+
+function numLeavesInTree(binaryTree) {
+
+  let numLeaves = 0;
+
+  function _preOrderTraversal(root) {
+
+    if(!root) {
+      return;
+    }
+
+    numLeaves += 1;
+    _preOrderTraversal(root.left);
+    _preOrderTraversal(root.right);
+
+  }
+
+  _preOrderTraversal(binaryTree.root);
+  return numLeaves;
+
+}