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 didn't 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 I'm not stupid or a bad programmer. even though I don't 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

I'm 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

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" in the 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.