Looking at the commit log [1], this code started in May 2013. That might explain why you might be thinking - why not async|bluebird|Promises? It looks like bluebird didn't start until Sept 2013 [2].

[1] https://github.com/douglascrockford/RQ/commits/master [2] https://github.com/petkaantonov/bluebird/commits/master?page...