detect-monorepo

A tiny, zero-dependency helper that walks upward from a starting directory to decide whether it sits inside a JS/TS monorepo workspace.

It recognises:

• pnpm workspaces (pnpm-workspace.yaml)
• npm, yarn, and bun workspaces (workspaces field in package.json)
• Rush (rush.json)

It is intended for tools that need a cheap pre-check before loading heavier monorepo tooling — for example, isolate-package is only useful inside a monorepo, so consumers like firebase-tools-with-isolate can call detectMonorepo first and only pay the cost of loading isolate-package when a workspace is actually detected.

Install

pnpm add detect-monorepo
# or
npm install detect-monorepo

Usage

import { detectMonorepo } from "detect-monorepo";

const info = detectMonorepo();
// or pass an explicit start directory:
const info = detectMonorepo("/path/to/some/package");

if (info) {
  console.log(`Monorepo detected at ${info.rootDir} (kind: ${info.kind})`);
} else {
  console.log("Not inside a monorepo");
}

The function returns either null or:

type MonorepoInfo = {
  /** Absolute path to the monorepo workspace root. */
  rootDir: string;
  /** Which workspace marker was found. "workspaces" covers npm/yarn/bun. */
  kind: "pnpm" | "workspaces" | "rush";
};

Behaviour

• Walks upward from startDir (default: process.cwd()) until a workspace marker is found, a directory containing a VCS-root marker (.git, .hg, or .svn) is reached, or the filesystem root is reached. The VCS-bearing directory is itself checked for workspace markers before traversal stops, so a workspace root and a repo root may coincide. For projects inside a VCS working copy the boundary prevents the walk from straying outside the repository into unrelated workspace markers higher up the filesystem; outside a VCS working copy there is no such bound and the walk continues to the filesystem root.
• Returns the first match found while walking upward.
• A package.json that cannot be parsed, or one whose workspaces field isn't an array or a { packages: string[] } object, is treated as "no workspace marker here" — the walk continues upward.
• Requires Node.js 20 or newer.

License

MIT © Thijs Koerselman