What are dirname and filename in Node.js?

Direct answer

__filename is the absolute path of the current module file; __dirname is its directory — CommonJS globals for resolving relative paths; in ESM use import.meta.url with fileURLToPath instead.

Before ESM, __dirname was the standard way to load config.json next to your script regardless of cwd. import.meta.url is the ESM equivalent tied to the module URL.

	CommonJS	ESM
File path	__filename	fileURLToPath(import.meta.url)
Directory	__dirname	path.dirname(fileURLToPath(import.meta.url))
Available	In .cjs / type absent	Not defined — use meta.url pattern

cwd vs __dirname — process.cwd() changes if user cd's before node; __dirname always follows the file location.
path.join — join(__dirname, 'data', 'file.json') avoids manual slash bugs (Q24).
Bundlers — may rewrite paths at build time; server runtime uses real filesystem paths.

dirname-esm.mjs

import path from "node:path";
import { fileURLToPath } from "node:url";

const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);

const configPath = path.join(__dirname, "config", "default.json");
console.log(configPath);