funcmt

Run a plain function on a worker_threads worker by serializing it with fn.toString() — no separate worker file to write and maintain.

The catch with fn.toString() is that the worker has no idea where the function originally lived, so any require(...) / import(...) inside it — relative paths or bare package names — breaks. funcmt parses the function's source with the TypeScript compiler API and rewrites those specifiers to absolute paths before shipping the function off, so they resolve correctly regardless of the worker's own location.

Install

npm install funcmt

typescript (^5) is a peer dependency — funcmt uses its compiler API to parse function source. Requires Node.js >=22.

Usage

import { runInWorker } from "funcmt";

function heavyWork(n: number) {
  const { fib } = require("./fib"); // resolved to an absolute path before running
  return fib(n);
}

const result = await runInWorker(heavyWork, [40], import.meta.url);

fromFile (the third argument — import.meta.url or __filename) anchors any relative specifiers found inside fn.

API

runInWorker(fn, args, fromFile)

Runs fn(...args) on a new worker thread and resolves with its return value (or rejects with whatever it throws).

• fn — a plain function, arrow, or async function. Object/class method shorthand (foo() {}) doesn't stringify to a valid standalone expression and isn't supported.
• args — arguments passed to fn inside the worker.
• fromFile — the file fn was written in, used to resolve its relative/bare specifiers.

rewriteSpecifiersToAbsolute(code, fromFile)

The lower-level primitive runInWorker is built on. Parses code, finds every require(...), dynamic import(...), and static import/export module specifier, and rewrites each one to an absolute location resolved from fromFile:

• require(...) specifiers become raw absolute filesystem paths.
• import(...) / import ... from specifiers become file:// URLs — Node's ESM loader doesn't accept a bare Windows-style absolute path.
• Node builtins (node:fs, path, ...) and specifiers that can't be resolved (e.g. a variable instead of a string literal) are left untouched.

Limitations

• Only functions whose toString() is a valid standalone expression can be run — no object/class method shorthand.
• Specifiers must be string literals; require(someVariable) can't be statically rewritten.
• No built-in worker pool, timeout, or cancellation — runInWorker spawns one worker per call.

Development

See .github/CONTRIBUTING.md.

License

MIT