A simple algorithm for traversing a binary search tree in-order without using recursion.

treeTraversal.js

function traverseTreeInOrder(tree) {

  // In the real world, we'd need a more robust solution of storing
  // and looking up object IDs. But JavaScript doesn't make that easy.
  const visitedNodes = {};

  const stack = [ tree ];
  while (stack.length > 0) {
    let node = stack.pop();
    if (!node) {
      continue;
    }

    if (!visitedNodes[node.id]) {
      stack.push(node.right);
      stack.push(node);
      stack.push(node.left);
      visitedNodes[node.id] = true;
    } else {
      visitNode(node);
    }

  }
}

function visitNode(node) {
  console.log(node.id);
}

const SampleTree = {
  id: 7,
  left: {
    id: 4,
    left: {
      id: 2,
      left: { id: 1 },
      right: { id: 3 }
    },
    right: {
      id: 6,
      left: { id: 5 }
    }
  },
  right: {
    id: 8,
    right: {
      id: 10,
      left: { id: 9 },
      right: {
        id: 11,
        right: { id: 12 }
      }
    }
  }
};

traverseTreeInOrder(SampleTree);