{
  foo: { bar: 1 },
  date: new Date('2014-01-01T00:00:00Z'),
  regex: /foo/i,
  bla: undefined
}

{
  foo: { bar: 1 },
  date: { $date: '2014-01-01T00:00:00Z' },
  regex: { $regex: 'foo', $options: 'i' },
  bla: { $undefined: true }
}

var mongoify = require('mongoify')

var obj = { date: new Date('2014-01-01T00:00:00Z') }

mongoify(obj)

console.log(JSON.stringify(obj))