Keep track of the overall completion of many disparate processes

 Track complex hierarchies of asynchronous task completion statuses. This is
 intended to give you a way of recording and reporting the progress of the big
 recursive fan-out and gather type workflows that are so common in async.
 .
 What you do with this completion data is up to you, but the most common use
 case is to feed it to one of the many progress bar modules.
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 Node.js is an event-based server-side JavaScript engine.