Build ESM Browser libraries from Deno Monorepos

March 20, 2022


Suppose you are building applications for browsers. Suppose that you have a design in mind for your project, warranting many modules. You want to:

  • author & test these modules with some amount of independence from one another
  • develop each of these modules concurrently

A monorepo is often the solution for these needs.

With the advent of deno and esm, this is an increasingly common pattern. Defining "monorepo" is out of scope, but for simplicity sake I will make an assertion that a monorepo hosts independent projects that may or may not reference one another. Consider deno_std. deno_std has http and fs concerns baked into the same repository, and is thus considered a monorepo from this point forward.

Problem Statement

  • There is no well established workflow to compile individual browsers libraries in a monorepo from TypeScript to ESM with deno1


  • TypeScript libraries are not consumable by browsers using ESM without pre-processing, but a pre-processing workflow is not well known for deno oriented monorepos

1 Assuming no bundlers are applied


  • imports work in development using deno
  • imports work in production using browsers
  • sourceMaps work in production using browsers
  • TypeScript compiles to ESM, on a per file file basis
  • ESM artifacts can be uploaded directly to HTTP static serving hosts, with no further processing required


What options do we have to to publish TypeScript ESM libraries using deno? So much of the conversation in Deno is around getting pre-existing JS resources into deno. What we want is the inverse--to get great work from a deno starting project into browsers!

  1. Bundle libaries with deno bundle
  • ❌ This strategy is undesirable, as
    • Libraries are meant to offer incremental bits of functionality. Bundling implicitly turns libraries into fully baked, statically linked SDKs, which do not compose or dedupe well.
    • Bundling is more fit for applications, versus libraries
      • In some cases, bundling may be acceptable. We will operate under the assumption that bundling is not appropriate for library distribution.
  1. Publish .ts source code, just as does
    • ❌ This strategy is undesirable, as
      • Browsers cannot import() TypeScript natively
      • CDNs cannot read TypeScript modules as ESM, compiled on the fly
        • does offer partial support for this, but has macro inhibitors:
          • uses a bundler under the hood, which is intrinsically wasteful. we seek to not bundle source code for this use case.
          • uses npm as it's module entrypoint, versus github or an arbitrary static file server
  2. Run tsc and upload the artifacts
    • ❌ This strategy is undesirable, as
      • We want to use deno, not tsc
      • importMaps do not work
  3. Use Deno.emit(...), and rewire imports
    • ✅ Let's explore this below

Solution - deno esm browser (deb) & Deno.emit(...)

To jump directly to the code solution, you can see the build function in the deb module, checked into the rad repository.

Consider that using deno often implies use of the following features:

  • importMaps
  • TypeScript

These are powerful tools, and are both commonly used in the deno space. They are not required for use in deno, they are simply common, and thus should be supported. If we are building browser-ready modules, we want to support these features.

How could it work? If you'd like to follow along interactively, you can see a fully featured demo here, for those who want to try it out.

Let's consider some input source:

// foo/mod.ts
import { bar } from "bar/mod.ts";
export const foobar = () => `foo${bar()}`;

Assume that bar/* is resolved via an import map, e.g. importMap.development.json during dev. I may run deno --import-map importMap.development.json test foo/mod.test.ts to test foo/mod.ts. Deno reads and tests this browser module just fine!

Getting this TypeScript ESM to JavaScript ESM is easy. Call Deno.emit(...) on this module, and you will get:

// foo/mod.ts.js
import { bar } from "bar/mod.ts";
export const foobar = () => `foo${bar()}`;
# sourceMap=...

Not bad. What problems exist?

  • bad resolution: bar/mod.ts will not be resolved by a browser. bar/mod.ts is not a relative sibling to foo/mod.ts
  • wrong asset type: bar/mod.ts is still a TS file :/
    • Interestingly, bar/mod.ts.js was generated, as it is part of foo/mod.ts's import graph!

How can we solve these problems? Let's focus on module resolution first. Suppose we know what our HTTP server or CDN base URL is going to be upfront. For instance, I host static assets on If I want to publish to my own static file server, I know my hostname and pathname for where I want to upload these assets!

Just like I am using importMap.development.json to resolve bar/* in development, I can author a importMap.production.json, and simply re-write imports post-compile. Let me update importMap.production.json to map { "bar/": "" }

In other words Demo.emit(...) |> rewriteImports("importMap.production.json") yields something like:

// foo/mod.ts.js
import { bar } from "";
export const foobar = () => `foo${bar()}`;
# sourceMap=...

Excellent! Pop on a .js extension during import re-writes, and we will have solved all outstanding problems:

// foo/mod.ts.js
import { bar } from "";
export const foobar = () => `foo${bar()}`;
# sourceMap=...

After pushing the build artifacts up, we can now try importing the ESM JS assets.

Demo on ObservableHQ

observable demo

So how did we actually compile it? Rather than repeat documentation in other places, instead allow me to guide you via links, end to end:

  • I've published a build function, that accepts ESM module filenames to compile
    • Deno.emit finds each file in the import graph, and emits an ESM module & .map file next to the associated .ts file
      • You can use the outDir option, to move all of these ESM assets to a directory of your choice, versus polluting your source code directories
    • Post compilation, we visit every emitted ESM source file and ask
      • What imports are used here?
      • For every import,
        • is there a mapping for this import in the provided production importMap?
          • if so, re-write the import, and flush it to disk
  • I've plugged the build function into a project where I want to publish foo & bar libraries, and it emits all the ESM to the .build directory.

I simply the rsync'd the ESM to my static file server, and the ObservableHQ demo above works!


Why use Deno at all here?

Deno is the superior toolkit & runtime for developing browser libraries.

  • typescript: zero config needed for authoring browser libraries. zero. open a typescript file, starting using browser APIs with great TS types.
  • performance: deno uses a highly performant compiled module caching strategy that covers the entire runtime, not just the project in play. this means normal TS performance on first run, and high performance on all incremental changes
  • testing: No JSDom, no jest. Deno.test(...) is all you need. It has browser runtime primitives baked in, versus needing to polyfill/shim in a virtual browser env, a la JSDom/happy-dom.

What if my TypeScript imports external Deno modules

Right--this project only converts your Deno source code to ESM--not the whole world's! If there are external, browser friendly, TypeScript ESM modules you want to tap into, you could:

Option 1: Add git submodules for your dependencies.

  • import your dependencies via local importMap references, the


  • publish the compiled 3rd party modules in the same structure as referenced in development

This option isn't great, as git submodule adds moderate complexity to all workflows. However, conceptually, it's quite simple if you're willing to add submodule functionality to your project.

Option 2: An ESM server, such as emit_esm_server

In this strategy, you must deploy a server to produce ESM. However, it does have a huge benefit--you can import TS files directly from your JavaScript, and it just works. Slow, probably needs loads of optimizations contingent on use case, but works.

# docker-compose.yaml
  image: cdaringe/deno_emit_server
        memory: 120M