Kitson P. Kelly avatar

A recovering 7 foot tall cactus

The Usual Suspects

The Javascript eco-system is varied these days and JSR is attempting to be a package registry up for the challenge. I wanted to share my experience of taking the middleware framework I originally wrote for Deno and making it work under Node.js, Bun and Cloudflare Workers.

A suspect lineup featuring an anthropomorphic Chinese dumpling, a cartoon brontosaurus, a construction worker in hi-viz, and a turtle wearing flight goggles

The story so far

I wrote oak back in 2018 as part of the early Deno community. I was making a lot of contributions to Deno and Ry Dahl and I started regularly chatting about things. He felt that Node.js really didn't start to take off until Express came along, and Deno really needed something like that.

Back in 2018, Deno's HTTP server was implemented in TypeScript and was part of the Deno std library and Deno Deploy was a couple years in the future. Eventually Deno implemented a native HTTP server under the Deno.serveHttp() API and then eventually the Deno.serve() API.


I originally had Node.js compatibility in oak via dnt, in fact I was still with Deno and oak was sort of a guinea pig for dnt. dnt plugged some gaps, like it allowed module substitution and made polyfilling a lot of APIs trivial. It also allowed me to run my test suite under Node.js and check for runtime specific issues.

The problem with dnt is that it builds a separate version of the package for publishing on npm. When migrating to JSR, you have a single code base of which JSR makes it so that the package can be consumed via npm compatible package managers. Having a single version of code you are publishing is great, but currently the developer experience is a bit clunky.

That being said, it was relatively straight forward to refactor my code to support Node.js. I had chosen in oak quite a while ago to abstract the core HTTP server. This was originally due to the evolving nature of the Deno HTTP server, but once abstracted, it became a lot easier to implement other runtimes as well. So there is a http_server_node.ts which interfaces to the Node.js built-in http module. This really didn't change when moving to JSR.

What did need to change was dynamically importing the correct HTTP server abstraction at runtime, detecting the runtime environment. Previously dnt was swapping it out. The other thing that needed to change is that I needed to manually expose the web standard streams in Node.js into the global namespace.


While the surface API of Bun.serve() and Deno.serve() is quite different in ways, the whole lifecycle of how requests and responses are handled is pretty much the same (except for web sockets). This meant it was actually quite straight forward to implement http_server_bun.ts.

Cloudflare Workers

Cloudflare Workers doesn't have an HTTP server, instead, you export a fetch handler. This sorts of inverts the whole process. That meant that I could create a http_server_cw.ts or something like that. A while back I had a request for a .handle() method on the main class of oak, Application, that could be used to adapt to arbitrary HTTP servers or use cases without needing to implement the oak server abstraction.

Taking a look at Cloudflare Workers fetch handlers though, there was clearly a cleaner implementation I could do, and so I created the .fetch() method in Application which can be default exported:

import { Application } from "@oak/oak";

const app = new Application();
app.use((ctx) => {
  ctx.response.body = "hello world!";

export default { fetch: app.fetch };

There were a couple other challenges with Cloudflare Workers, where it appears to load all possible modules that it was able to statically analyze to make them available, even if they are only dynamically imported. This meant the top level imports of the Node.js builtin HTTP server were occurring, even though that code would never be called. I had to move the dynamic imports into the class implementations themselves to make the module loadable by the wrangler.

Still to do

The biggest gap at the moment is web sockets. oak supports web sockets on Deno CLI and Deno Deploy, but not elsewhere. Each runtime is quite a different story about it. Deno chose to adopt the web standard WebSocket with a basic upgrade flow that turns an http connection into a web standard WebSocket, Bun uses a handler mechanism with its own ServerWebSocket interface, Node.js would need to use something like ws and Cloudflare Workers uses a WebSocketPair.

Cloudflare Workers should be the easiest, as it is actually pretty close to what Deno does current. Both Node.js and Bun should be possible, but are likely a lot more work and will have a sort of convoluted testing path at the moment.

Cloudflare Workers provide some additional context, like the environment variables and the ability to wait until promises which affect the lifetime of the request handler. These aren't exposed in oak.

Sending files via the .send() is not supported expect on Deno CLI and Deno Deploy. This functionality is quite coupled to Deno's specific file APIs to be able to support things like byte ranges and the like, let alone more general stuff like generation of etags based on the file's state. Even then, oak is still using deprecated Deno APIs because of its age. Likely I will have to abstract away the APIs like I have the HTTP server APIs.

There is a lot of cruft in oak, mostly due to it heritage, having been through multiple generations of Deno HTTP server implementations. Some of the server abstractions have leaked into other bits of code and unwinding them is likely to be complex, but something that should eventually get done.

Also, Supporting HTTPS on Node.js would be a nice to have.

Keyser Söze

The main challenges with getting code to run "everywhere" breaks down into three areas:

  • Runtime specific APIs
  • Filling gaps in standard APIs
  • Runtime specific implementations

Runtime specific APIs

For oak, I abstracted these away, giving myself a single API to have to manage internally, while each implementation could deal with vagaries of the runtime specific APIs.

The APIs you expose to your users should be as universal as possible and not conditional on the runtime environment. This just gets really messy for your users and makes integrations of your API not portable. In oak, for .listen() I standardized on the Deno specific options for things like TLS. That means for other implementations, I have to conform those options to match the runtime reality.

Filling gaps in standard APIs

All the runtimes implement a lot of the web standard APIs, though depending on your needs, there are gaps. For example, Bun refuses to implement URLPattern (oak doesn't use it, but it a good example), but has most of the other web standard APIs.

The best way to address this is to conditionally load and expose polyfills. One that is present in Deno, but almost no where else is the DOM ErrorEvent which oak uses.

Also, specifically for Node.js, some of the web standard APIs are available, they are just not exposed in the global scope. Like for example the web streams that oak uses. Conditionally loading and exposing them in the global scope tends to work.

I would recommend depending on web standard APIs of which there is at least one reference implementation in a browser and a server side runtime. Depending on something that isn't quite fully baked yet, or isn't a standard, is going to lead to a lot of heart ache.

Also, loading polyfills should be feature detected in the runtime environment. Assumptions about that Node.js doesn't support X or Bun doesn't support Y are short sighted. All the runtimes are evolving quite quickly these days and all are generally committed to shipping standard APIs, so what assumptions you make today might not hold true tomorrow.

Runtime specific implementations

If dealing with the runtime specific APIs is more complex than a couple branches of code, and you have chosen to implement an abstraction of some kind, then conditional dynamic imports are the way to go. In oak, the HTTP server abstraction is conditionally dynamically loaded.

One thing to note is that Cloudflare Workers will statically analyze the dependency graph of your code and the wrangler will load all modules it detects, this means that if you are loading some built-in Node.js modules, even dynamically, they can't be at the top level, as the wrangler will fail to successfully execute those modules. Instead you will need to move them as dynamic imports deeper in the code.

In the end

JSR takes care of the "packaging" part of the code, so each runtime can be fed modules in a way that can be best managed with the runtime, but JSR does not address these "functional" level challenges, and that is where you have to step in.

Addressing those functional challenges is possible though, and while the overall developer experience of supporting multiple runtimes needs to improve with JSR, it is really cool that I can, with a single code base, provide a strongly typed middleware server framework that runs natively on Deno CLI, Deno Deploy, Node.js, Bun and Cloudflare Workers, providing users of oak relatively easy access to run their applications they have built with oak on it.