Typing JavaScript with JSON Schema

January 3rd, 2017

Typing JavaScript with JSON Schema

I had an idea. It’s half-formed, probably not a good one, but hey, I thought it’d be fun to share.


In the last couple of months, while I’m patiently for the latest Battletech game to become available, I’d been working on my port of FullThrust. Because I want it to be a educational experience, I’ve decided to go full JavaScript for both the frontend and backend. And I really mean, full-frontal, no hold barred, modern, bleeding-edge JavaScript.

The result so far? On the game front, the progress has been abysmal. But oh boy did I learn a lot.

I will spare you the long list of frameworks that the project iterated through. Suffice to say that the state engine at the core of the application is converging toward a global store — the penultimate incarnation was using Redux, and the current one is using VueX.

Since all those stores use JavaScript objects to both represent the main store as well as the actions that alter it, one of the very latest yak to step on the barber chair was typing systems. Because, I’m at least sufficiently self-aware to know I’m a scatterbrain who will never remember from one file to the next if I settled for

let movement = {
    heading:  1,
    velocity: 1,
    coords:   [ 0, 5 ],


let movement = {
    bearing: 1,
    speed:   1,
    coords:  [ 0, 5 ],

At the very least I have to find a comfortable way to document my data structures. And if I can make my program automatically check that what I’m passing around remotely look like what it’s supposed to, hey, that’d be wondertastic.

Good enough won’t be left alone

As it happens, there are two big contenders (that I know of) in the JS ecosystem for typed systems. TypeScript and FlowJS. TypeScript is a whole superset of JavaScript, while Flow is strictly about type checking. Both are nifty by their own right, but they are not eeeexactly scratching my itch. After all that time spent embracing the latest, purest, uncut EcmaScript, I’m not sure I’m ready to veer toward TypeScript. And as for Flow, I tried to see how well it’d work with Vue and Webpack and, urgh, I think I reached the number of transpilations I can live with.

So what does that leave me with?

A mad scheme arises

Well, that leaves me with something that’s already in my toolkit: JSON Schema. Sure, it’s a little bit verbose, but that’s another perfectly cromulent way to express types. And it has some advantages over the TypeScript and Flow alternatives.

For one, it’s language agnostic, so can probably be used more-or-less straight for the OpenAPI or RAML web service the game will have. Or even be used to create equivalent types in, say, Perl using [JSON::Schema::AsType][].

It also allow for more sophisticated types. Sure, with TypeScript or Flow you can say that the property foo is a number. But using the latest JSON Schema, you can not only set ranges, but you can also make it dependent on other properties of the object.

var schema = {
  "properties": {
    "thrust": {
      "type":    "number",
      "minimum:  0,
      "maximum": { "$data": "1/engine_rating" }
    "engine_rating": { "type": "number" }

YAGNI-assured overkill? Perhaps. But so much power… it is alluring.

What it looks like

So, how would type-checking variables and stuff would look like with that kind of scheme? I’m still playing with it and haven’t generated anything generic yet, but here is what my current implementation in that game of mine looks like.

First, there is the Game/Schema/index.js module that encapsulates both the schema itself, and the validation functionality.

For the JSON schema itself, I’m leveraging ajv and, to sweeten the process, my own json-schema-shorthand:

import Ajv       from 'ajv';
import shorthand from 'json-schema-shorthand';

let schema = {
    id: "http://aotds.babyl.ca/schema.json",
    definitions: {
        movement: { 
            id: "#movement",
            object: {
                velocity:        "number!",
                heading:         "number!",
                coords:          'object!',
                trajectory:      'object',
                remaining_power: "number",
        "ship_navigation": {
            id: '#ship_navigation',
            "object": {
                heading:  'number!',
                velocity: 'number!',

let ajv = new Ajv();

ajv.addSchema( shorthand( schema ) );

For the validation, I basically want a way to wrap values and functions such that I’ll get squeaks of outrage if they don’t produce what’s expected of them.

// everything's better with a lodash of salt
import _ from 'lodash';

// logging stuff
import StackTrace from 'stacktrace-js';
import Bunyan     from 'bunyan';

let logger = Bunyan({ name: "aotds", src: true, level: 'debug' });

function _validate_type( type, data ) {
    let v = _.partial( 
        ( typeof type === 'object' )
            ? type 
            : { '$ref': 'http://aotds.babyl.ca/schema.json#' + type }

    let res = v(data);

    if(!v(data)) {
        StackTrace.get().then( trace => {
            // first 2 stack items are for
            // _validate_type and validate_type, so
            // not really interesting

            // filter out library stuff to cut on the noise
            trace = trace.filter( t => ! /node_modules/.test(t.fileName) );

            logger.warn( { 
                schema_error: ajv.errors,
                definition:   type,
            }, 'schema validation' )

    return data;

export default
function validate_type(type) {

    return input => {
        if( typeof input === 'function' ) {
            return (...args) => 
                _validate_type( type, input.apply(null,args) );

        return _validate_type( type, input );

And, well, that’s it. And its use is not too obtrusive either. For example, where I had

function ship_calculate_movement( ship, orders = {} ) {
    // lotsa stuff goes here

I can now do

import _t from 'Game/Schema';

let ship_calculate_movement = _t('movement')( ( ship, orders = {} ) => {

    _t('ship_navigation')( ship );

    // lotsa stuff goes here

and I’ll get big fat warnings if ship_calculate_movement returns something that is not fitting the movement type, or if it’s being fed an unnavigable ship. And while it’s not as pretty as

function ship_calculate_movement ( ship: ShipNavigation, orders = {} ) : Movement { 

it’s 100% pure JavaScript and doesn’t require any transhenaniganifactions. I can live with that.

Of course, I’m already see a lot of room for tweaks and improvements. For example, it’d be easy to have a configuration knob to throw errors when encountering malformed data. And to make the logging mechanism pluggable. Or allow the type checking to become a straight bypass for production and/or need for speed.

But that’s something for tomorrow. For the time being, I have a game to write. In the short, oh so terribly short time I have before the next yak jumps me.