Understanding the 'Module Wrapper Function' and CommonJS vs ESM

Node.js wraps CJS modules in a function to provide local variables like __dirname; ESM uses a top-level lexical scope and supports asynchronous loading.

In CommonJS, your code is wrapped: (function(exports, require, module, __filename, __dirname) { ... }). This is why these variables exist without being defined. ES Modules (ESM) do not have this wrapper, which is why __dirname is missing. ESM is static, allowing for better 'Tree Shaking' (removing unused code), whereas CJS is dynamic. Senior devs must understand that ESM is now the standard for Node.js (v14+), though CJS is still pervasive in legacy packages.