Add more comments to tree DFS algorithm.

Author: trekhleb

src/algorithms/tree/depth-first-search/depthFirstSearch.js

@@ -1,52 +1,76 @@
 /**
- * @typedef {Object} Callbacks
- * @property {function(node: BinaryTreeNode, child: BinaryTreeNode): boolean} allowTraversal -
- *   Determines whether DFS should traverse from the node to its child.
+ * @typedef {Object} TraversalCallbacks
+ *
+ * @property {function(node: BinaryTreeNode, child: BinaryTreeNode): boolean} allowTraversal
+ * - Determines whether DFS should traverse from the node to its child.
+ *
  * @property {function(node: BinaryTreeNode)} enterNode - Called when DFS enters the node.
+ *
  * @property {function(node: BinaryTreeNode)} leaveNode - Called when DFS leaves the node.
  */
 
 /**
- * @param {Callbacks} [callbacks]
- * @returns {Callbacks}
+ * Extend missing traversal callbacks with default callbacks.
+ *
+ * @param {TraversalCallbacks} [callbacks] - The object that contains traversal callbacks.
+ * @returns {TraversalCallbacks} - Traversal callbacks extended with defaults callbacks.
  */
 function initCallbacks(callbacks = {}) {
-  const initiatedCallback = callbacks;
+  // Init empty callbacks object.
+  const initiatedCallbacks = {};
 
+  // Empty callback that we will use in case if user didn't provide real callback function.
   const stubCallback = () => {};
-  const defaultAllowTraversal = () => true;
+  // By default we will allow traversal of every node
+  // in case if user didn't provide a callback for that.
+  const defaultAllowTraversalCallback = () => true;
 
-  initiatedCallback.allowTraversal = callbacks.allowTraversal || defaultAllowTraversal;
-  initiatedCallback.enterNode = callbacks.enterNode || stubCallback;
-  initiatedCallback.leaveNode = callbacks.leaveNode || stubCallback;
+  // Copy original callbacks to our initiatedCallbacks object or use default callbacks instead.
+  initiatedCallbacks.allowTraversal = callbacks.allowTraversal || defaultAllowTraversalCallback;
+  initiatedCallbacks.enterNode = callbacks.enterNode || stubCallback;
+  initiatedCallbacks.leaveNode = callbacks.leaveNode || stubCallback;
 
-  return initiatedCallback;
+  // Returned processed list of callbacks.
+  return initiatedCallbacks;
 }
 
 /**
- * @param {BinaryTreeNode} node
- * @param {Callbacks} callbacks
+ * Recursive depth-first-search traversal for binary.
+ *
+ * @param {BinaryTreeNode} node - binary tree node that we will start traversal from.
+ * @param {TraversalCallbacks} callbacks - the object that contains traversal callbacks.
  */
 export function depthFirstSearchRecursive(node, callbacks) {
+  // Call the "enterNode" callback to notify that the node is going to be entered.
   callbacks.enterNode(node);
 
-  // Traverse left branch.
+  // Traverse left branch only if case if traversal of the left node is allowed.
   if (node.left && callbacks.allowTraversal(node, node.left)) {
     depthFirstSearchRecursive(node.left, callbacks);
   }
 
-  // Traverse right branch.
+  // Traverse right branch only if case if traversal of the right node is allowed.
   if (node.right && callbacks.allowTraversal(node, node.right)) {
     depthFirstSearchRecursive(node.right, callbacks);
   }
 
+  // Call the "leaveNode" callback to notify that traversal
+  // of the current node and its children is finished.
   callbacks.leaveNode(node);
 }
 
 /**
- * @param {BinaryTreeNode} rootNode
- * @param {Callbacks} [callbacks]
+ * Perform depth-first-search traversal of the rootNode.
+ * For every traversal step call "allowTraversal", "enterNode" and "leaveNode" callbacks.
+ * See TraversalCallbacks type definition for more details about the shape of callbacks object.
+ *
+ * @param {BinaryTreeNode} rootNode - The node from which we start traversing.
+ * @param {TraversalCallbacks} [callbacks] - Traversal callbacks.
  */
 export default function depthFirstSearch(rootNode, callbacks) {
-  depthFirstSearchRecursive(rootNode, initCallbacks(callbacks));
+  // In case if user didn't provide some callback we need to replace them with default ones.
+  const processedCallbacks = initCallbacks(callbacks);
+
+  // Now, when we have all necessary callbacks we may proceed to recursive traversal.
+  depthFirstSearchRecursive(rootNode, processedCallbacks);
 }