Error: Route.get() requires callback functions - node.js

Question

After I added an app-api file and followed the instructions in the book: Simon Holmes Getting MEAN with Mongo, Express, Angular, and Node.
I restart the application and it shows error:
image
How to fix it?
file app.js:
var express = require('express');
var path = require('path');
var http = require('http');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
require('./app_api/models/db');
//require('./app_server/models/db');
var routes = require('./app_server/routes/index');
var routesApi = require('./app_api/routes/index');
var users = require('./app_server/routes/users');
var app = express();
// view engine setup
app.set('views', path.join(__dirname, 'app_server', 'views'));
app.set('view engine', 'jade');
// 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('/users', users);
app.use('/api', routesApi);
// catch 404 and forward to error handler
app.use(function(req, res, next) {
var err = new Error('Not Found');
err.status = 404;
next(err);
});
// error handler
app.use(function(err, req, res, next) {
// set locals, only providing error in development
res.locals.message = err.message;
res.locals.error = req.app.get('env') === 'development' ? err : {};
// render the error page
res.status(err.status || 500);
res.render('error');
});
module.exports = app;
file index.js in app_api\routes
var express = require('express');
var router = express.Router();
var ctrlLocations = require('../controllers/locations');
var ctrlReviews = require('../controllers/reviews');
// locations
router.get('/locations', ctrlLocations.locationsListByDistance);
router.post('/locations', ctrlLocations.locationsCreate);
router.get('/locations/:locationid', ctrlLocations.locationsReadOne);
router.put('/locations/:locationid', ctrlLocations.locationsUpdateOne);
router.delete('/locations/:locationid', ctrlLocations.locationsDeleteOne);
// reviews
router.post('/locations/:locationid/reviews', ctrlReviews.reviewsCreate);
router.get('/locations/:locationid/reviews/:reviewid',
ctrlReviews.reviewsReadOne);
router.put('/locations/:locationid/reviews/:reviewid',
ctrlReviews.reviewsUpdateOne);
router.delete('/locations/:locationid/reviews/:reviewid',
ctrlReviews.reviewsDeleteOne);
module.exports = router;
file locations in app_api\controllers:
var mongoose = require('mongoose');
var Loc = mongoose.model('Location');
var sendJsonResponse = function(res, status, content) {
res.status(status);
res.json(content);
};
module.exports.locationsCreate = function (req, res) {
sendJsonResponse(res, 200, {"status" : "success"});
};
file reviews.js in app_api\controllers
var mongoose = require('mongoose');
var Loc = mongoose.model('Location');
var sendJsonResponse = function(res, status, content) {
res.status(status);
res.json(content);
};
module.exports.reviewsCreate = function (req, res) {
sendJsonResponse(res, 200, {"status" : "success"});
};

Answer 1

The stack trace tells you exactly where the problem is:
ap_api\routes\index.js:7
i.e. Line 7 of index.js, which is this:
router.get('/locations', ctrlLocations.locationsListByDistance);
The error says that the callback is undefined. Here the 'callback' is ctrlLocations.locationsListByDistance, which doesn't appear to be in your locations controller.