example.js

import { mapLimit, auto, constant } from 'async'
import request from 'request'
import analyzer from './analyze'
import logger from './log'

const data = [
  'http://example.com/item/0'
  'http://example.com/item/1'
  'http://example.com/item/2'
  'http://example.com/item/3'
]

function fetch (opts, callback) {
  const { url } = opts
  request(url, callback) // this will do a simple GET of the url
}

function parse (opts, callback) {
  const { fetch } = opts
  let data = null
  try {
    data = JSON.parse(fetch.body)
  } catch (err) {
    return callback(err)
  }
  callback(null, data)
}

function process (item, callback) {
  const block = {
    url: constant(item),
    fetch: ['url', fetch],
    data: ['fetch', parse],
    analyze: ['data', (opts, cb) => analyze(opts.data, cb),
    log: ['analyze', log]
  }
  auto(block, callback)
}

function done (err, results) {
  if (err) return console.error(err)
  results
    .map(r => r.analyze) // log the analysis results
    .map(r => console.log(r))
}

// This calls `process` for each item in data, maximum 2 at a time
// Then passes the results to done, as a `results` array
// If any item returns an error, then the first error is send to `err` and no results
mapLimit(data, 2, process, done)