Lorenzo von Matterhorn Lorenzo von Matterhorn - 1 year ago 128
Node.js Question

MEAN Stack App Structure

I used the express-generator npm to create a boilerplate for a MEAN Stack app and it worked perfectly but I didn't understand the purpose of a few files.

For example:

The package.json contained the following code:

"script":{"start": "node ./bin/www"}

The app contained a folder called
which contained a file called
which contained the code below:

#!/usr/bin/env node

* Module dependencies.

var app = require('../app');
var debug = require('debug')('myapp:server');
var http = require('http');

var port = normalizePort(process.env.PORT || '3000');
app.set('port', port);

var server = http.createServer(app);

server.on('error', onError);
server.on('listening', onListening);

function normalizePort(val) {
var port = parseInt(val, 10);

if (isNaN(port)) {
// named pipe
return val;

if (port >= 0) {
// port number
return port;

return false;

function onError(error) {
if (error.syscall !== 'listen') {
throw error;

var bind = typeof port === 'string' ?
'Pipe ' + port :
'Port ' + port;

// handle specific listen errors with friendly messages
switch (error.code) {
case 'EACCES':
console.error(bind + ' requires elevated privileges');
console.error(bind + ' is already in use');
throw error;

function onListening() {
var addr = server.address();
var bind = typeof addr === 'string' ?
'pipe ' + addr :
'port ' + addr.port;
debug('Listening on ' + bind);

Now Im not sure whats the purpose of this because I removed all this code above and added the following lines in my app.js file where my server is:

var port = process.env.PORT || 8080;
console.log("Listening on port " + port)

By replacing all that code with only two lines I was able to run the server and display a view using routes. Thats how I have been developing all my node/express apps for a while and they have worked fine.

Can anyone explain what was the point of all that code because I am not sure what it does? And why would be need it when we can simply replace it with 2 lines? It just seems very confusing and unnecessary.

Answer Source

Inside of your package.json file, the line "script":{"start": "node ./bin/www"} tells node where to go to start your application.

The code that you removed includes error checking and validates that the server is listening on a normalized port and running.

The line: server.on('error', onError); creates an event listener that is bound to the onError method. When an error is detected, the onError() method is called and executed, throwing the error.

Similarly, the server.on('listening', onListening); creates an event listener that is bound to the onListening method. When the server is actually listening on a normalized port the onListening() method is called and executed.

The big difference between the generated code and your code, is that it provides error handling, while yours does not. Error-handling is absolutely essential and should not be removed if you intend to push your project into a live environment.

It provides a way for you application to give you better error information (aiding in problem resolution/debugging), and handle problems without completely falling apart. If your application does not start up or crashes, but you have not built-in error-handling or reporting, it can make finding the problem quite tedious at times.

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