Promises in recursive asynchronous functions

Question

I want to take a chrome bookmark and find its parents, all the way up to the parent bookmark folder.

The function below works for getting the first parent of the given bookmark.

function getParent(bookmark) {
  var promise = new Promise(function(resolve, reject) {
    chrome.bookmarks.get(bookmark.parentId, function(nodes) {
      resolve(nodes[0]);
    });
  });
  return promise;
}

I'm having trouble getting all of the bookmarks parents. The function below doesn't work but it should show what I'm looking for.

function getParents(bookmark, parents) {
  var parents = parents || [];
  var promise;

  getParent(bookmark).then(function(parent) {
    if (parent.parentId == '0') {
      parents.push(parent);
      promise = Promise.resolve(parents);
    } else {
      parents.push(parent);
      getParents(parent, parents);
    }
  });
  return promise;
}

I guess my real question is: how do you get promises to work in recursive asynchronous functions?

Or if you have a better way of doing this, that works too.

Answer 1

Simply return the promise returned by .then and either return the final value from the .then callback, or another promise:

function getParents(bookmark, parents) {
  var parents = parents || [];

  return getParent(bookmark).then(function(parent) {
    parents.push(parent);
    return parent.parentId == '0' ? parents : getParents(parent, parents);
  });
}

See http://www.html5rocks.com/en/tutorials/es6/promises/#toc-chaining