source: imaps-frontend/node_modules/asynckit/lib/iterate.js

var async = require('./async.js')
  , abort = require('./abort.js')
  ;

// API
module.exports = iterate;

/**
 * Iterates over each job object
 *
 * @param {array|object} list - array or object (named list) to iterate over
 * @param {function} iterator - iterator to run
 * @param {object} state - current job status
 * @param {function} callback - invoked when all elements processed
 */
function iterate(list, iterator, state, callback)
{
  // store current index
  var key = state['keyedList'] ? state['keyedList'][state.index] : state.index;

  state.jobs[key] = runJob(iterator, key, list[key], function(error, output)
  {
    // don't repeat yourself
    // skip secondary callbacks
    if (!(key in state.jobs))
    {
      return;
    }

    // clean up jobs
    delete state.jobs[key];

    if (error)
    {
      // don't process rest of the results
      // stop still active jobs
      // and reset the list
      abort(state);
    }
    else
    {
      state.results[key] = output;
    }

    // return salvaged results
    callback(error, state.results);
  });
}

/**
 * Runs iterator over provided job element
 *
 * @param   {function} iterator - iterator to invoke
 * @param   {string|number} key - key/index of the element in the list of jobs
 * @param   {mixed} item - job description
 * @param   {function} callback - invoked after iterator is done with the job
 * @returns {function|mixed} - job abort function or something else
 */
function runJob(iterator, key, item, callback)
{
  var aborter;

  // allow shortcut if iterator expects only two arguments
  if (iterator.length == 2)
  {
    aborter = iterator(item, async(callback));
  }
  // otherwise go with full three arguments
  else
  {
    aborter = iterator(item, key, async(callback));
  }

  return aborter;
}