Proof of concept for inlining web worker code with esbuild with shared dependencies deduplicated.

Try it

yarn
yarn example # check the JS console to see that the main thread and worker code executed successfully.
cat example/bundle.ts # notice that the code from shared.ts only appears once

How it works

The idea is to use a two-step process:

First, take advantage of code splitting to factor out the common dependencies between the main entrypoint and the worker, which produces three "chunks", that look sort of like:

main.js

import { SharedThing } from "./shared.js";
// src/main.ts
import { createWorker } from "inlined-worker!./worker"; // NOTE: we intentionally skipped this import in the first step.
// We'll come back to it in the next step.
console.log("main", new SharedThing().id);
createWorker();

worker.js

import { SharedThing } from "./shared.js";
// src/worker.ts
function doSomething() {
  console.log("worker", new SharedThing().id);
}
doSomething();

shared.js

// src/shared.ts
var SharedThing = class {
  constructor() {
    this.id = Math.random();
  }
};
export { SharedThing };

We don't write these chunks to disk -- our plugin configures ESBuild to generate them in memory.

Next, we manually combine these three chunks into a single one as follows:

1. From shared.js, emit code that: a. wraps the original source in a function, __sharedModuleFn. b. evaluates that function immediately to provide the actual exports from shared.js (e.g. SharedThing in the example above), storing them as __sharedModuleExports
2. From worker.js, emit code that creates a __workerFn function that accepts an object holding the exports from the shared module, and then code that uses Function.toString() to assemble both of these, at runtime, into a string containing the full source of the worker. This string is used to create an object URL with which to instantiate the worker.