What are the binary tree traversal orders?

The three depth-first orders differ only in when the current node is processed. That single change produces very different sequences, so naming the order precisely matters. Level order is breadth-first and uses a queue instead of the call stack.

function inorder(node, output = []) {
  if (node === null) return output;
  inorder(node.left, output);
  output.push(node.value);
  inorder(node.right, output);
  return output;
}

The orders and their uses

• Preorder: copy a tree, serialize it, or explore roots before leaves
• Inorder: sorted output from a binary search tree
• Postorder: delete a tree, evaluate an expression tree, compute subtree results
• Level order: shortest path in an unweighted tree or breadth-first processing

function inorderIterative(root) {
  const output = [];
  const stack = [];
  let node = root;
  while (node !== null || stack.length > 0) {
    while (node !== null) {
      stack.push(node);
      node = node.left;
    }
    node = stack.pop();
    output.push(node.value);
    node = node.right;
  }
  return output;
}