how to fix this error when running my app - node.js

Question

I'm new to NodeJS,I'm just trying to create a restfull API, but when I try to execute the code by running (npm run dev),it's throwing an error.There are many questions like this one, but no answer could figure out what is wrong in my code.
the error is : app.use() requires a middleware function
this is my index.js
const express = require("express");
const app = express()
const departmentRouter = require("../src/routes/department")
app.use(express.json())
app.use(departmentRouter)
const port = process.env.PORT || 3000;//local machine port 3000
app.listen(port, () => (`Server running on local machine port ${port} 🔥`));
this is the model :
const mongoose = require("mongoose");
const validator = require('validator')
const Department = mongoose.model('Department', {
name: {
type: String,
required: true,
}
,
email: {
type: String,
required: true,
trim: true,
lowercase: true,
validate(value) {
if (!validator.isEmail(value)) {
throw new Error('Invalid email!')
}
}
}
,
createdBy: {
type: String,
default: 'SYS_ADMIN'
}
,
updatedBy: {
type: String,
default: 'SYS_ADMIN'
}
,
createdAt: {
type: Date
// ,
// default: Date.getDate()
}
,
updatedAt: {
type: Date
// ,
// default: Date.getDate()
},
isDeleted: {
type: Boolean,
default: false
}
})
module.exports = Department
this is routes file (department.js)
const express = require("express");
const router = express.Router();
const Department = require("../models/department")
router.get("/v1/departments", async (req, res) => {
try {
const departments = await Department.find({ isDeleted: false })
if (!departments) {
return res.status(404).send()
}
res.status(200).send()
} catch (error) {
res.status(500).send(error)
}
});
router.get("/v1/department/:id", async (req, res) => {
//test if the department exist => by id
const _id = req.params._id
try {
const depatment = await Department.findByid(_id, { isDeleted: false })
if (!depatment) {
return res.status(404).send()
}
res.status(200).send(depatment)
} catch (error) {
res.status(500).send(error)
}
});
router.post("/v1/department", async (req, res) => {
try {
const department = new Department(req.body) //this param is for testing the post methode by sending request from postman
await department.save()
} catch (error) {
res.status(500).send(error)
}
});
router.put("/v1/department/:id", async (req, res) => {
//especially useful when testing the put method using postman
const updates = Object.keys(req.body)
const allowedUpdates = ['name', 'email']
const isValideOperations = updates.every((update) => allowedUpdates.includes(update))
if (!isValideOperations) {
return res.status(400).send({ error: 'Invalid updates' })
}
try {
const _id = req.params.id
const department = await Department.findByIdAndUpdate(_id)
if (!department) {
return res.status(404).send()
}
res.send(department)
} catch (error) {
res.status(500).send(error)
}
})
//Safe delete by updating the field isDeleted to true
router.delete('/v1/department/:id', async (req, res) => {
try {
const _id = req.params.id
const department = await Department.findByIdAndUpdate(_id, { isDeleted: true })
if (!department) {
return res.status(400).send()
}
res.status(200).send(department)
} catch (error) {
res.status(500).send(error)
}
})

Answer 1

In the department.js file, you need to export the router:
module.exports = router