npm install router

• index.js file

const express = require("express");
const app = express();
const {userRouter} = require("./routes/user");

app.use(express.json());

app.use("/user",userRouter);

app.listen(3000);

• user.js file

const express = require("express");
const userRouter = express.Router();

// all the rest of the logic, then at the end of the file 

module.exports = {
	userRouter
}

What is circular dependency?