NodeJS. Cannot read property of undefined - node.js

Question

Have a problem with using another method in class.
I'm working with express framework and have to use Ecma Script 6.
When I try to use method in the same class I have an error:
Cannot read property 'generatePassword' of undefined
TypeError: Cannot read property 'generatePassword' of undefined
at createUser (E:\OpenServer\domains\testNode\app\controllers\users.js:5:13)
at Layer.handle [as handle_request] (E:\OpenServer\domains\testNode\node_modules\express\lib\router\layer.js:95:5)
at next (E:\OpenServer\domains\testNode\node_modules\express\lib\router\route.js:131:13)
at Route.dispatch (E:\OpenServer\domains\testNode\node_modules\express\lib\router\route.js:112:3)
at Layer.handle [as handle_request] (E:\OpenServer\domains\testNode\node_modules\express\lib\router\layer.js:95:5)
at E:\OpenServer\domains\testNode\node_modules\express\lib\router\index.js:277:22
at Function.process_params (E:\OpenServer\domains\testNode\node_modules\express\lib\router\index.js:330:12)
at next (E:\OpenServer\domains\testNode\node_modules\express\lib\router\index.js:271:10)
at Function.handle (E:\OpenServer\domains\testNode\node_modules\express\lib\router\index.js:176:3)
at router (E:\OpenServer\domains\testNode\node_modules\express\lib\router\index.js:46:12)
My code:
Route: /routes/users.js
const express = require('express');
const router = express.Router();
const users = require('../app/controllers/users');
router.get('/', users.createUser);
module.exports = router;
Controller: /app/controllers/users.js
class Users
{
createUser(req, res)
{
this.generatePassword();
res.json({msg: "The method return user data" });
}
generatePassword()
{
return 'The method return new password';
}
}
module.exports = new Users();
Main App file: /app.js
const express = require('express');
const path = require('path');
const favicon = require('serve-favicon');
const logger = require('morgan');
const cookieParser = require('cookie-parser');
const bodyParser = require('body-parser');
const index = require('./routes/index');
const users = require('./routes/users');
const app = express();
// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'hbs');
// 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('/', index);
app.use('/users', users);
// catch 404 and forward to error handler
app.use(function(req, res, next) {
let 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;
package.json
{
"name": "testnode",
"version": "0.0.0",
"private": true,
"scripts": {
"start": "node ./bin/www"
},
"dependencies": {
"body-parser": "~1.15.2",
"cookie-parser": "~1.4.3",
"debug": "~2.2.0",
"express": "~4.14.0",
"hbs": "~4.0.1",
"morgan": "~1.7.0",
"serve-favicon": "~2.3.0"
}
}
Help me please, thank you.

Answer 1

In the line that causes the error, the object this is not what you think it is. To solve this, replace the following:
router.get('/', users.createUser);
with:
router.get('/', users.createUser.bind(users));
This will make sure when eventually that function is called by the router.get implementation, it will be called with the this object set to users.