When there are tons of users, “RangeError: Maximum call stack size exceeded” occurs and it dies in the middle.

async.eachSeries users, (user, next) ->
  user.save (error)->
    return next()
, (error) ->
  // ...

Wrapping with process.nextTick or setImmediate as shown below resolves it:

async.eachSeries users, (user, next) ->
  process.nextTick ->
    user.save (error)->
      return next()
, (error) ->
  // ...

---

Reference Information

・Node.js - Maximum call stack size exceeded - Stack Overflow

・‘Maximum call stack size exceeded’ using async.forEachLimit · Issue #75 · caolan/async

That’s all from the Gemba.