Async with a null value returned

async_test.js

var async = require('async')
  , _ = require('underscore');

var array = [1,2,3,4,5];

async.map(array, function(item, cb) {
  if (item === 3)
    return cb();
  else 
    return cb(null, item*2);
}, function(e, newArray) {
  console.log('End error : ', e);
  console.log('End array : ', newArray);
  console.log('Compact   : ', _.compact(newArray));
});

/*
  End error :  undefined
  End array :  [ 2, 4, undefined, 8, 10 ]
  Compact   :  [ 2, 4, 8, 10 ]
*/