ES Module for Web Extensions


It is pretty easy if you’re using manifest v2 🎇 after 2021/05/18 (Firefox 89 release).

For background page, popup, ..etc:

<script type="module" src="./background.js"></script>

For content scripts:

Please read the Conclusion section below.

Some outdated content about how to support Firefox before Firefox 89 has removed. Please refer to the git history of this article if you’re interested.

About node_modules

To use npm packages, notice that snowpack is not strong enough as the webpack to handle so many cases, it may fail to build your dependencies but please have a try!

Update on 05/19/2020: I’m going to pack dependencies by webpack and distribute them in UMD format. I wrote a custom typescript transformer for converting imports into a UMD access. Here is a template project to use it with webpack.


In Mar 2019, @pika/web (renamed to snowpack) released.
I read the article few months later, and learned the concept of unbundled development.

I felt interested and inspired by that idea. That’s so cool! You can run ES Modules directly in the browser and enjoy the benefits of it.

That time I’m working on a browser extension called Maskbook, and I found it’s possible to try the unbundled development in our project. Browser extensions are pre-downloaded as a ZIP file to the computer therefore it’s no need to worry if the unbundled project will send too many HTTP requests and slow down the app. All files are loaded by local IO.

Problem to solve

During the development of Maskbook, we encountered many problems with webpack chunk splitting.

Update on 05/19/2020: Please check out neutrino-webextension, it’s a Web Extension preset for Webpack that support chunk splitting or dynamic import.

Update on 10/06/2020: We’re now using webpack-target-webextension. It resolved all the problems. neutrino-webextension mentioned above is based on this package too.

There’re 5 entries.

  • Content script: run in the isolated high privileged sandbox in the target page
  • Background page: run as the “server” of the extension
  • Options page: a normal webpage page but in the -extension:// protocol
  • Popup: another normal webpage
  • Workers: do some heavy works

It is a common practice to share dependencies (like React) in different entries to reduce the size.
Webpack has built-in support for this so we turn it on.
Then the nightmare comes.

  • webpack wants to load chunks by injecting the <script> tag, which will let the chunk content goes into another environment instead of the isolated environment of content scripts. (Resolved by a hack on HTMLScriptElement.prototype.src)
  • The Webpack-way of loading import('...') breaks our code.
  • … and so many bugs caused by the chunk splitting

Chunk splitting is designed for normal webpages, not for WebExtension (which have it’s own sandbox, protocol, CSP, etc…), and since it cause so many bugs in Maskbook thus decided to totally close the chunk splitting completely.
But we still need a way to share dependencies.

But how to resolve it

There are some nature ideas to resolve this problem.

  • Manually splitting the dependencies (this idea is too boring and I’m not going to talk about them. (but I’m still considering it before this experiment has succeeded))
  • Other module loaders like AMD (😕Nah, like living in the 19th century)
  • ESModule.

(✔️) ESModule YES

First, checkout the browser compatibility of static import and dynamic import at

Nice. But that’s for the normal webpage. Let’s try it in WebExtension.

(✔️) Background page

Firstly I tried the following code.

import "./shared.js";

And failed with Uncaught SyntaxError: Cannot use import statement outside a module.

So the file need to be declared as ESModule. Renaming the file to .mjs, not working.

Looking for the document of manifest.json, but nowhere mentioned ESModule.
There is a field in the background section: manifest.json/ We can declare a HTML file for the background.


<script type="module" src="/background.js"></script>

And that’s working!

Browser supporting:

Chrome Firefox Firefox for Android
import { } from '' ✔️* ✔️*
import('') ✔️* ✔️*

*: Need to use a HTML file

(✔️) Content script

Same as the background page, tried to run import './shared.js' directly.

And failed with Uncaught SyntaxError: Cannot use import statement outside a module again.

Unfortunately, solutions for the background page doesn’t work for the content script because content script runs in other webpages and don’t have their own HTML.

Then I tried:


Failed with a network error. Chrome tries to load the script from instead of chrome-extension://extension-id/content.js. Hmm, interesting 🤔, so I changed it to


Chrome only: You need to set web_accessible_resources in the manifest so the JS file is accessible in normal webpage. Set that in your own cautions.

And it works! 🎇

Browser supporting:

Chrome Firefox Firefox for Android
import { } from ''
import('') ✔️* ✔️*?

*: Need to wrap with chrome.runtime.getURL()

(✔️❌ partly works) Use npm modules in the browser

A GitHub organization @pikapkg/ has already made a solution for this.

A Future Without Webpack: snowpack installs npm packages that run natively in the browser. Do you still need a bundler?

Experiment in production: DimensionDev/Maskbook:feature/experiment-pika and there are 2(or 3) problems.

Import path

Browsers only know how to load an absolute URL or relative URL and the .js cannot be omitted. But we already used to import('lib') or import('./code'). That’s an easy problem to resolve. Snowpack provides a solution to do it but I’m using tsc to compile Maskbook thus I wrote a TypeScript custom transformer and load it by ttypescript to transform the import path.

Note on import './file.json'

JSON import is rewritten to data:application/javascript,export default {"json": "content"}

JSON import is rewritten to const json = JSON.parse("JSON file content")

Update on 05/19/2020: Sorry for the mistake. The data url import is banned by the CSP in the extension environment. I switch to inline the JSON, see the example at the document of @magic-works/ttypescript-browser-like-import-transformer.

Note on “folder import”

When writing import './sth' in TypeScript, this import declaration may means

  • import './sth.js'
  • import './sth.jsx'
  • import './sth.ts'
  • import './sth.tsx'
  • import './sth/index.js'
  • import './sth/index.jsx'
  • import './sth/index.ts'
  • import './sth/index.tsx'

When writing an path transform plugin, don’t forget to cover all the cases!

Update on 05/19/2020: Good news, you don’t need to reinvent the wheel. @magic-works/ttypescript-browser-like-import-transformer also supports folder import.

💥 No Tree-shaking

This whole section about tree-shaking is out-dated.

Snowpack provides tree-shaking now.

I also implements tree shaking for @magic-works/ttypescript-browser-like-import-transformer! (Link to documentation)

It seems impossible to use tree-shaking with @pika/web. It doesn’t scan your code to drop all unused dependencies. It tries to transform all packages in dependencies to ESModule in your package.json.

Full packages of lodash-es, @material-ui/core and @material-ui/icons are generated with a horrifying size.

🤔 Doesn’t know how to resolve it yet.

❌ Cannot omit optional dependencies

It’s a common pattern to do in the npm packages.

try {
// faster but need native binding
} catch {
// slower

snowpack currently cannot config to omit some of the packages.
And it will try to build everything even it is an optional dependency. This makes the compilation process slow even not
available to work.

Conclusion (in Nov 2020)


I tried to pack all our dependencies by snowpack. It failed. Some of our dependencies let Rollup throws (e.g. Web3.js), some of our dependencies let snowpack out of response. I moved to another route: if Snowpack can pack this dependencies, I covert the import route to “/web_modules”. If not, I rewrite the import statement into a global variable access.

I made a TypeScript custom transformer to do the AST transformation.

It will transform the code

import a from "a";

into this

const a = _import(globalThis["a"], ["default"], "a", "globalThis.a", false).default;
import { _import } from "";

Feel ironic huh? I want to import libraries as ES Modules, but finally my solution is using global variables.

What ever, the dependency problem is resolved. My final solution doesn’t have snowpack in it.
I wrote a custom Webpack loader will drop all normal codes and add something like:

import { something } from "some_library";
__using("some_library", { something });

to track what dependencies webpack should bundle, then __using will set those references into global variables.

Next step: HMR

After it is being able to work, the next step is support HMR. It is possible to HMR in ES Module but I’m not willing to investigate more time in inventing tool chain. I gave up and return to the webpack.


Here is some useful resources.



Packages and repos


Jack Works

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