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Archetype is a runtime type-casting library. Its purpose is to compose types from existing types in a way that's easy to write and document.

const { ObjectId } = require('mongodb');
const moment = require('moment');
// `Person` is now a constructor 
const Person = new Archetype({
  name: 'string',
  bandId: {
    $type: ObjectId,
    $required: true
  createdAt: {
    $type: moment,
    $default: () => moment()
const test = new Person({
  name: 'test',
  bandId: '507f191e810c19729de860ea'
test.bandId; // Now a mongodb ObjectId 
test.createdAt; // moment representing now 

If casting fails, archetype throws a nice clean exception:

try {
  Person({ // <-- calling with `new` is optional 
    name: 'test',
    bandId: 'ImNotAValidObjectId'
} catch(error) {
  error.errors['bandId'].message; // Mongodb ObjectId error 

Archetypes are composable, inspectable, and extendable via extends.