TypeError: Router.use() requires middleware function

Question

I am developing a REST API through node using express. When I try to run my application, I get this error:

TypeError: Router.use() requires middleware function but got a Object
    at Function.use (/Users/....../Desktop/NodeProjects/MyWebsite/node_modules/express/lib/router/index.js:458:13)
    at EventEmitter.<anonymous> (/Users/......./Desktop/NodeProjects/MyWebsite/node_modules/express/lib/application.js:219:21)
    at Array.forEach (native)
    at EventEmitter.use (/Users/........./Desktop/NodeProjects/MyWebsite/node_modules/express/lib/application.js:216:7)
    at Object.<anonymous> (/Users/............./Desktop/NodeProjects/MyWebsite/app.js:32:5)
    at Module._compile (module.js:398:26)
    at Object.Module._extensions..js (module.js:405:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)
    at Function.Module.runMain (module.js:430:10)

Below is my code:

app.js Code

  var express        = require('express');
  var http           = require('http');
  var path           = require('path');
  var favicon        = require('serve-favicon');
  var logger         = require('morgan');
  var cookieParser   = require('cookie-parser');
  var bodyParser     = require('body-parser');
  var nodemailer     = require('nodemailer');
  var url            = require('url');

  var routes         = require('./routes/index');
  var contacts       = require('./routes/contacts');

  var app            = express();

  // view engine setup
  app.set('views', path.join(__dirname, 'views'));
  app.set('view engine', 'jade');
  app.set('port', process.env.PORT || 3000);

  // uncomment after placing your favicon in /public
  //app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
  app.use(logger('dev'));
  app.use(bodyParser.json());
  app.use(bodyParser.urlencoded({ extended: false }));
  app.use(cookieParser());
  app.use(express.static(path.join(__dirname, 'public')));
  app.use('/', routes);
  app.use('/contacts', contacts);

  // catch 404 and forward to error handler
  app.use(function(req, res, next) {
    var err    = new Error('Not Found');
    err.status = 404;
    next(err);
  });

  app.get('/contacts',function(request, response){
    var get_params = url.parse(request.url, true).query;
    if (Object.keys(get_params).length == 0)
    {
      response.setHeader('content-type', 'application/json');
      response.end(JSON.stringify(contacts.list()));
    }
    else
    {
     response.setHeader('content-type', 'application/json');
     stringify(contacts.query_by_arg(get_params.arg, get_params.value));
   }
 }); 


  http.createServer(app).listen(app.get('port'), function() {
   console.log('Express server listening on port ' + app.get('port'));
 });


  module.exports = app;

Code of Contacts.js

var fs = require('fs');

//Read Json file
function read_json_file() {
   var file = './data/contacts.json';
   return fs.readFileSync(file);
}

//Parse the the file da
exports.list = function() {
   return JSON.parse(read_json_file());
};

exports.query = function(number) {
   var json_result = JSON.parse(read_json_file());
   var result = json_result.result;
   for (var i = 0; i < result.length; i++) {
      var contact = result[i];
      if (contact.primarycontactnumber === number)    {
         return contact;
      }
   }
   return null;
};

exports.query_by_arg = function(arg, value) {
   var json_result = JSON.parse(read_json_file());
   var result = json_result.result;
   for (var i = 0; i < result.length; i++) {
      var contact = result[i];
      if (contact[arg] === value) {
         return contact;
      }
   }
   return null;
};

exports.list_groups = function() {
  var json_result = JSON.parse(read_json_file());
  var result       = json_result.result;
  var resultArray  = [];
  for (var i = 0; i < result.length; i++) {
   var groups = result[i].groups;
   for (var index = 0; index < groups.length; index++) {
      if (resultArray.indexOf(groups[index]) === -1) {
         resultArray.push(groups[index]);
      }
   }
}
return resultArray;
};

exports.get_members = function(group_name) {
   var json_result = JSON.parse(read_json_file());
   var result = json_result.result;
   var resultArray = [];
   for (var i = 0; i < result.length; i++) {
      if (result[i].groups.indexOf(group_name) > -1) {
         resultArray.push(result[i]);
      }
   }
   return resultArray;
};

When I comment out the app.use('/contacts', contacts); line in my app.js then it works fine. Otherwise I get this issue. I have tried a lot to dig into the reason behind it but have failed. Can any one help me with this please.

Answer 1

As per the docs, app.use is for registering middleware; your contacts.js doesn't contain any middleware, it just contains the helper functions you use inside the actual /contacts route.

As you noted, commenting it out makes everything work just fine. There's no need to do anything else :)

You'll also want to move your 404 handler after your routes, otherwise it'll be run before every route, hence the 404s you see. See the error handling docs for more info.