Static typing is becoming very popular in the JavaScript community. While I'm sure TypeScript is a brilliant feat of engineering, I believe it's a product that gives you a 20% advantage for 80% extra work. In my mind, TypeScript is a supplement for mature JS dev teams with a lot of unavoidable complexity in their business logic. For most JavaScript apps, runtime type casting is all you need.

What's Wrong With TypeScript?

Here's a simple server in TypeScript:

import * as express from 'express';
import { json } from 'body-parser';

const app = express();

class Test {
  public a: string;
}'/', (req, res) => {
  res.send((<Test> req.body).a.substr(0, 1)); // <-- Danger!


Look carefully at the POST route. Let's say you hit the POST route with an HTTP request where the body has a string property 'a':

Everything works as expected. But what if 'a' is a number?

Everything goes boom. Experienced TypeScript developers shouldn't find this surprising, but I'm sure most TypeScript developers have gotten bit by this at some point.

Is this an obscure edge case? I'd argue not. Static typing was designed for the good old days when network interaction was an afterthought in programming. If this where 1975 and you were working on Unix at Bell Labs, your colleagues' work would usually be in the same program as yours. In other words, you'd usually interface with your colleagues' code by calling a function, and your compiler could check your types were correct.

Fast forward to 2016. Today, most organizations divide their code into smaller programs that talk over networks (microservices, etc.). If you were working on Photoshop in the 90's, you had one giant monolith and your compiler could check your UI code's types matched with lower level function signatures. Today, most developers work on code that's that talks to countless services written in completely different languages on many different machines. No compiler will help you ensure that the data coming into your program from the network is correct.

Runtime Type Casting with Archetype

At Booster, we decided to rewrite our LoopBack-based API earlier this year. LoopBack helped us get a product out the door fast, but was riddled with too many embarrassing bugs for us to continue using beyond the MVP phase.

The primary problem we wanted to solve was data validation. The HTTP layer was not a problem because Express' rich feature set is already borderline overkill for us. Concurrency (AKA callback hell, promise hell, etc.) was not a problem either because of co. What we lacked was a good way to make sure the data coming in to the API was correct. We evaluated a bunch of options:

  • TypeScript didn't solve our problem
  • Joi had a hopelessly ugly API and made it difficult to add custom types
  • The various JSON schema modules didn't do a good job of supporting nested types and custom types
  • Mongoose makes introspection difficult and is too closely coupled to MongoDB to use effectively in the browser.

Naturally, the solution was to write our own, and archetype was born. Archetype is an extensible and nestable library for runtime type casting. Let's say you want to define what makes a valid Person:

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()

Person is now a constructor that throws an error if and only if the given data is invalid.

try {
  new Person({
    name: 'test',
    bandId: 'ImNotAValidObjectId'
} catch(error) {
  error.errors['bandId'].message; // Mongodb ObjectId error

If the data is valid, archetype will cast all the object's properties into the given types: 'name' will be a string, 'bandId' will be a fully fledged MongoDB ObjectId, and 'createdAt' will be a moment.js object.

There's nothing special about the moment or MongoDB ObjectId types. Writing your own custom type is as simple as writing a class. For example, suppose you wanted to work around the issue where technically any string of length 12 is a valid parameter to the MongoDB ObjectId constructor:

const mongodb = require('mongodb');

class MyObjectId extends mongodb.ObjectId {
  constructor(v) {
    if (typeof v !== 'string' || !/^[0-9a-f]{24}$/.test(v)) {
      throw new Error(`Invalid value ${v}`);

const Person = new Archetype({
  name: 'string',
  bandId: {
    $type: MyObjectId,
    $required: true

If the MyObjectId constructor throws an error, new Person() will throw an error as well. Because of this, archetypes are nestable too.

const Person = new Archetype({
  name: 'string'

const Band = new Archetype({
  name: 'string',
  people: [Person]

new Band({
  name: "Guns N' Roses",
  people: [{ name: 'Axl Rose' }]

Archetype works particularly well with destructuring assignments. Instead of writing this:

const { name, bandId } = req.body;
if (name == null) {
  throw new Error('Name is required');
if (typeof name !== 'string') {
  throw new Error('Name must be a string');
if (bandId == null) {
  throw new Error('BandId is required');
try {
  bandId = new ObjectId(bandId);
} catch (error) {
  throw new Error('Invalid ObjectId');

You can just write:

const Person = new Archetype({
  name: { $type: 'string', $required: true },
  band: { $type: ObjectId, $required: true }

const { name, bandId } = new Person(req.body);

Archetype makes handling incoming data from HTTP or from user-provided options a breeze. There's no more excuse to bury yourself in walls of if statements.


If you're worried about type safety in JavaScript, you need a runtime casting library, even if you're using Flow or TypeScript. When your data comes in over the network, you have no way to check your data at compile time. In this case, static type checks are at best unnecessary and at worst lull you into a false sense of security. If you're looking to get serious about data integrity, check out archetype and let us know what you think on GitHub or in comments!

Found a typo or error? Open up a pull request! This post is available as markdown on Github
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