Test Framework

A minimal Node.js test framework that discovers test files and runs each one in a dedicated worker thread.

Architecture

The framework is split into two main components: the main thread that orchestrates the work, and the worker threads that execute the tests.

index.mjs — Main thread

• Uses jest-haste-map to crawl the project and discover all *.test.js files.
• Spawns one Worker per file, passing the file path via workerData.

Resolving Worker Paths with new URL()

To tell the Worker constructor which script to run, we use new URL('./worker.mjs', import.meta.url).

• How it works: This creates a full, absolute URL to worker.mjs. It uses the location of the current module (import.meta.url) as the base and resolves the relative path (./worker.mjs) from there.
• Why it's important for ESM: In modern Node.js (ES Modules), file paths are treated as file:// URLs. This is the standard way to get a file path relative to the current module, replacing the __dirname variable from older CommonJS modules. It differs from the path module, which manipulates path strings rather than creating URL objects.

worker.mjs — Worker thread

• Receives filePath from workerData.
• Reads the file and prints its path and content in a single atomic console.log — guaranteeing the output is never interleaved with output from sibling workers.

How Node.js Worker Threads Work

A Worker Thread is a separate, parallel stream of execution. Each worker:

1. Runs in Isolation: It has its own V8 engine instance, its own event loop, and its own memory.
2. Prevents Blocking: It executes its script (worker.mjs) without blocking the main thread. This is crucial for performance, as it allows the main application to remain responsive while the workers perform CPU-intensive tasks in the background.
3. Receives and Sends Data: It's initialized with data from the main thread via the workerData property, and sends results or data back to the main thread using parentPort.postMessage(), which the main thread receives via worker.on('message', ...) events.

How it works

node index.mjs
  └─ Main thread
       ├─ jest-haste-map discovers all *.test.js files
       └─ for each file → new Worker("./worker.mjs", { workerData: { filePath } })
            └─ Worker thread
                 ├─ reads workerData.filePath
                 ├─ fs.readFileSync(filePath)
                 └─ single console.log(path + content)  ← atomic, no interleaving

Running

node index.mjs