File system and path

Reading and writing files is one of the most common things a Node program does — loading configuration, persisting data, processing logs, generating reports. Node's `fs` module provides the file system operations, and the `path` module provides the tools to build and manipulate file paths correctly. The two are almost always used together. The `fs` module offers three flavours of nearly every operation: a promise-based API under `fs/promises`, a traditional callback API, and synchronous variants ending in `Sync`. For modern code the promise-based API is the default, because it composes with async/await and, as the event loop lesson established, keeps the single thread free while the disk works. The `path` module exists because file paths are deceptively tricky. Different operating systems use different separators — forward slashes on Unix and macOS, backslashes on Windows — and concatenating path strings by hand produces code that breaks when moved between systems. `path` handles these differences correctly so you never hardcode a separator. There is also a subtle but important wrinkle in ES Modules: the `__dirname` and `__filename` variables that CommonJS provided automatically do not exist in ESM, and must be reconstructed from `import.meta.url`. Knowing this saves a confusing early error. Mastering file and path handling — reading and writing safely, building cross-platform paths, handling the inevitable errors like a missing file, and streaming large files rather than loading them whole — is foundational for almost any real Node application, from build tooling to web servers.