BFS(root)
  Pre: root is the node of the BST
  Post: the nodes in the BST have been visited in breadth first order
  q ← queue
  while root = ø
    yield root.value
    if root.left = ø
      q.enqueue(root.left)
    end if
    if root.right = ø
      q.enqueue(root.right)
    end if
    if !q.isEmpty()
      root ← q.dequeue()
    else
      root ← ø
    end if
  end while
end BFS

import Queue from '../../../data-structures/queue/Queue';
/**
 * @typedef {Object} Callbacks
 * @property {function(node: BinaryTreeNode, child: BinaryTreeNode): boolean} allowTraversal -
 *   Determines whether BFS should traverse from the node to its child.
 * @property {function(node: BinaryTreeNode)} enterNode - Called when BFS enters the node.
 * @property {function(node: BinaryTreeNode)} leaveNode - Called when BFS leaves the node.
 */
/**
 * @param {Callbacks} [callbacks]
 * @returns {Callbacks}
 */function initCallbacks(callbacks = {}) {
  const initiatedCallback = callbacks;  const stubCallback = () => {};
  const defaultAllowTraversal = () => true;  initiatedCallback.allowTraversal = callbacks.allowTraversal || defaultAllowTraversal;
  initiatedCallback.enterNode = callbacks.enterNode || stubCallback;
  initiatedCallback.leaveNode = callbacks.leaveNode || stubCallback;  return initiatedCallback;
}
/**
 * @param {BinaryTreeNode} rootNode
 * @param {Callbacks} [originalCallbacks]
 */export default function breadthFirstSearch(rootNode, originalCallbacks) {
  const callbacks = initCallbacks(originalCallbacks);
  const nodeQueue = new Queue();  // Do initial queue setup.
  nodeQueue.enqueue(rootNode);  while (!nodeQueue.isEmpty()) {
    const currentNode = nodeQueue.dequeue();    callbacks.enterNode(currentNode);    // Add all children to the queue for future traversals.    // Traverse left branch.
    if (currentNode.left && callbacks.allowTraversal(currentNode, currentNode.left)) {
      nodeQueue.enqueue(currentNode.left);
    }    // Traverse right branch.
    if (currentNode.right && callbacks.allowTraversal(currentNode, currentNode.right)) {
      nodeQueue.enqueue(currentNode.right);
    }    callbacks.leaveNode(currentNode);
  }
}