TypeError: Router.use() requires a middleware function but got a undefined at Function.use

Question

I am new to nodeJS and I was learning how to create a simple api app which connects to mongoDB.

I read a few blogs and studied a few simple projects, I started to make it on my own but there was a slight issue. I followed all the steps and even checked solving the issue on my own.

Though, I wasnt successful the issue comes in server.js when I link it with api.js and throws the following error

D:\Node Js\nodeJs\userStory\node_modules\express\lib\router\index.js:458
      throw new TypeError('Router.use() requires a middleware function but got a ' + gettype(fn))
      ^

TypeError: Router.use() requires a middleware function but got a undefined
    at Function.use (D:\Node Js\nodeJs\userStory\node_modules\express\lib\router\index.js:458:13)
    at Function.<anonymous> (D:\Node Js\nodeJs\userStory\node_modules\express\lib\application.js:220:21)
    at Array.forEach (<anonymous>)
    at Function.use (D:\Node Js\nodeJs\userStory\node_modules\express\lib\application.js:217:7)
    at Object.<anonymous> (D:\Node Js\nodeJs\userStory\server.js:26:5)
    at Module._compile (module.js:643:30)
    at Object.Module._extensions..js (module.js:654:10)
    at Module.load (module.js:556:32)
    at tryModuleLoad (module.js:499:12)
    at Function.Module._load (module.js:491:3)
    at Function.Module.runMain (module.js:684:10)
    at startup (bootstrap_node.js:187:16)
    at bootstrap_node.js:608:3

Would be helpful if anyone can have a look at it and guide me where I am going wrong.

//---Server.js
var express = require('express');
var bodyParser = require('body-parser');
var morgan = require('morgan');
var config = require('./config');
var mongoose = require('mongoose');


var app = express();

mongoose.connect(config.database,function(err){
	if(err){
		console.log(err);
	}else{
		console.log("Connected to a database");
	}
});


app.use(bodyParser.urlencoded({extended:true}));
app.use(bodyParser.json());
app.use(morgan('dev'));



app.use("/api",require('./app/route/api')(app,express));

app.get('*',function(req,res){
	res.sendFile(__dirname + '/public/views/index.html');
});

app.listen(config.port,function(err){
	if(err){
		console.log(err);
	}else{
		console.log("Listening on port "+config.port);
	}
});

//api.js
var user = require('../model/user');

var config = require('../../config');

var secretKey = config.secretKey;

module.exports = function(app,express){

    var api = express.Router();
    //passing the data to the DB

    api.post('/signup',function(req,res){
        var user = new User({
            name:req.body.name,
            username:req.body.username,
            password:req.body.password
        });//body here is the body parser for reading the value from the DB

        //Now saving it on the DB

        user.save(function(err){
            if(err){
                res.send(err);
                return;
            }
            res.json({message:'User has been created'});
        });
    });

};

Answer 1

There are different cases when this error comes.

In my case, I got this error when code was as below

app.js

const { getRouter } = require('./routes/getAPIs');

...
expressServer.use("/", getRouter);

other.js

const router = expressServer.Router();
router.get("/", getData);
module.exports = router;

Here the problem is using destructing way of import instead of normal way when destructuring way is not used(We used module.exports method).

Solution for this is remove {} around the import object.

app.js

const getRouter = require('./routes/getAPIs');

...
expressServer.use("/", getRouter);

Answer 2

You should export the router, not a function.

var api = express.Router();
//passing the data to the DB

api.post('/signup',function(req,res){
    var user = new User({
        name:req.body.name,
        username:req.body.username,
        password:req.body.password
    });//body here is the body parser for reading the value from the DB

    //Now saving it on the DB

    user.save(function(err){
        if(err){
            res.send(err);
            return;
        }
        res.json({message:'User has been created'});
    });
});
module.exports = api;

Change your app.js accordingly

app.use("/api",require('./app/route/api'));