Given a database that you want to retrieve from with async calls, and records in it that represent nodes of a tree (each of which has a 'children' array with containing ids of other node records), here's a way to walk that tree with callbacks. Saving this in case I later decide I need this, after all.

treegetting.js

var levelup = require('level');
var _ = require('underscore');

var TreeGetter = function TreeGetter(db, rootNodeId, childDepth, done) {
  this.treeGetState = {
    db: db,
    nodesToGet: 0,
    nodesGot: 0,
    errors: [],
    depthLimit: childDepth,
    tree: {},
    done: done
  };
};

TreeGetter.prototype.getNodeFromDb = function getNodeTreeFromDb(
  rootNodeId, depth, parent) {

  ++this.treeGetState.nodesToGet;

  this.treeGetState.db.get(rootNodeId, function getComplete(error, value) {
    this.receiveNode(error, depth, parent, value);
  }
  .bind(this));
};

TreeGetter.prototype.receiveNode = function receiveNode(
  error, depth, parent, node) {

  if (error) {
    this.treeGetState.errors.push(error);
    return;
  }

  ++this.treeGetState.nodesGot;

  if (parent) {
    debugger;
    // Replace the id in children with the node object.
    var childIndex = parent.children.indexOf(node.id);
    parent.children[childIndex] = node;
  }
  else {
    this.treeGetState.tree = node;
  }

  debugger;

  if (depth < this.treeGetState.depthLimit && 
    typeof node.children === 'object') {

    node.children.forEach(function getNodeChild(childId) {
      this.getNodeFromDb(childId, depth + 1, node);
    }
    .bind(this));
  }

  this.wrapUpIfComplete();
}

TreeGetter.prototype.wrapUpIfComplete = function wrapUpIfComplete() {
  debugger;
  if (this.treeGetState.nodesGot >= this.treeGetState.nodesToGet) {
    this.treeGetState.done(this.treeGetState.errors, this.treeGetState.tree);
  }
};

module.exports.getTree = function getTree(db, nodeId, childDepth, done) {
  var treeGetter = new TreeGetter(db, nodeId, childDepth, done);
  treeGetter.getNodeFromDb(nodeId, 0, null);
};