Options for nested `async.eachSeries`. In response to http://stackoverflow.com/questions/21184340/async-for-loop-in-node-js/21184964?noredirect=1#comment67220656_21184964

rename-inner-next.js

var async = require('async')
async.eachSeries(
  [ 1, 2, 3 ]
, function (key, next) { 
    async.eachSeries(
      [ 4, 5, 6 ]
    , function (key, innerNext) { // note the name change
        somethingAsync('something' + key, innerNext) // using the inner one
      }
    , function (innerErr) {
        next(innerErr) // note the call to the outer one so that an inner error bubbles out
      }
    )
  }
, function (err) {
    console.log('iterating done')
  }
)

separating-the-inner-one.js

var async = require('async')

function innerAsyncSeries(key, done) {
  async.eachSeries(
    [ 4, 5, 6 ]
  , function (key, next) { // note the name change
      somethingAsync('something' + key, next)
    }
  , function (err) {
      done(err) // note the call to the outer one so that an inner error bubbles out
    }
  )
}

async.eachSeries(
  [ 1, 2, 3 ]
, innerAsyncSeries
, function (err) {
    console.log('iterating done');
  }
)