what does fileURLToPath(import.meta.url) do?

Question

Okay so i was following a mern stack tutorial and the tutor wrote out some lines of code but didnt really explain them well. this is the code:

const path = require('path')
const {fileURLToPath} = require('url')

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

app.use("/assets", express.static(path.join(__dirname, "public/assets")));

Now Im not stupid or a bad programmer. even though i dont really have an explanation for what the code is doing, i have a pretty good idea. the problem is that when i run my database, i get this error:

SyntaxError: Identifier '__filename' has already been declared

which is crazy because i havent used '__filename' anywhere else in the code. when i try to change '__filename' to 'filename' then i get this error:

SyntaxError: Cannot use 'import.meta' outside a module

Im so confused. Please can someone just tell me what the code does and why i am getting these errors and also how to fix the errors.

Answer 1

check your package.json() if it is having a

"type":"module"

if it has then remove it

Answer 2

The __filename basically points to the current file you're writing the code in while __dirname gives you the parent folder of that current file. For example:

import { dirname } from "node:path";
import { fileUrlToPath } from "node:url";
const __filename = fileUrlToPath(import.meta.url);
const __dirname = dirname(__filename);

console.log('__filename:', __filename);
// --> /home/users/projects/example.js

console.log('__dirname:', __dirname);
// --> /home/users/projects

Writing this code is a waste of time, space and effort if you use CJS since Node.js already gives you these variables natively. So you can just go around and write __dirname or __filename anywhere in the code you want without having to declare them.

However, if you ESM, which you should, you won't have access to them. This seems to be the issue. You're redeclaring variables that node injects into CJS, not ESM, which means that you're using CJS, while your tutorial is using ESM. If you add "type": "module" to your package.json, that should resolve it.

If you're using a modern version of Node (or Deno, or Bun), however, defining __filename and __dirname are mostly redundant, becuase of the import.meta namespace, which includes dirname, filename, url, and a semi-standardized resolve mechanism.

Answer 3

This is happening because your code is built for esm, but you are working in commonjs. The above answer to remove "type": "Module" would cement it being commonjs. The other answer saying it is a waste of space and effort is only true if you are building a commonjs project. Ensure you know the differences, and recognize that esm is the new standard, and learning how to build an esm project may be more relevant to your career than just removing the module and __filename definition. Maybe not. I do not know your full intent. Happy coding!

Answer 4

What you are seeing there is the attempt to create the native constants __filename and __dirname (present only in commonJS modules) for an ESM module (.mjs file extension or "type":"module" ind package.json).

In ESM you have only import.meta.url pointing to the current file (like __filename with a "file://" prefix - which can be removed using fileURLToPath). Once you have __filename you can get __dirname easily.