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Express.js Interview Questions
It's a class of Node JS capable of emitting events. We do this by attaching the named events emitted by the object by using the function- eventEmitter.on().
The server initializes the Middleware, routes, and other application logic. On the other hand, the app contains all the business logic that the server-initiated routes will serve. This enables the encapsulation of the business logic from the application logic for smooth functioning.
The options available are given below :
 
* --sessions or –s for adding session report

* --hogan or –H for adding Hogan.js engine support

* --ejs or –e for adding EJS engine support

* --css <engine> or –c <engine> for adding style sheet support

* -jshtml or –J for adding JSHTML engine support

* --force or –f for forcing app generation on the directory which is non-empty.
We will follow the given steps to set up HTTP :
 
* Installing certbot
* Generating the SSL certificate by Certbot
* Allowing Express to serve the static files
* Confirming the domain
* Obtaining the certificate
* Setting up the renewal.
When running an Express app behind a reverse proxy, some of the Express APIs may return different values than expected. In order to adjust for this, the trust proxy application setting may be used to expose information provided by the reverse proxy in the Express APIs. The most common issue is express APIs that expose the client’s IP address may instead show an internal IP address of the reverse proxy.
 
The application setting trust proxy may be set to one of the values listed in the following table.

Type Value
Boolean

If true, the client’s IP address is understood as the left-most entry in the X-Forwarded-For header.

If false, the app is understood as directly facing the client and the client’s IP address is derived from req.socket.remoteAddress. This is the default setting.

IP addresses

An IP address, subnet, or an array of IP addresses and subnets to trust as being a reverse proxy. The following list shows the pre-configured subnet names:

  • loopback - 127.0.0.1/8, ::1/128
  • linklocal - 169.254.0.0/16, fe80::/10
  • uniquelocal - 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, fc00::/7

You can set IP addresses in any of the following ways:

app.set('trust proxy', 'loopback') // specify a single subnet
app.set('trust proxy', 'loopback, 123.123.123.123') // specify a subnet and an address
app.set('trust proxy', 'loopback, linklocal, uniquelocal') // specify multiple subnets as CSV
app.set('trust proxy', ['loopback', 'linklocal', 'uniquelocal']) // specify multiple subnets as an array

When specified, the IP addresses or the subnets are excluded from the address determination process, and the untrusted IP address nearest to the application server is determined as the client’s IP address. This works by checking if req.socket.remoteAddress is trusted. If so, then each address in X-Forwarded-For is checked from right to left until the first non-trusted address.

Number

Use the address that is at most n number of hops away from the Express application. req.socket.remoteAddress is the first hop, and the rest are looked for in the X-Forwarded-For header from right to left. A value of 0 means that the first untrusted address would be req.socket.remoteAddress, i.e. there is no reverse proxy.

Function

Custom trust implementation.

app.set('trust proxy', (ip) => {
  if (ip === '127.0.0.1' || ip === '123.123.123.123') return true // trusted IPs
  else return false
})

Enabling trust proxy will have the following impact :

* The value of req.hostname is derived from the value set in the X-Forwarded-Host header, which can be set by the client or by the proxy.  
* X-Forwarded-Proto can be set by the reverse proxy to tell the app whether it is https or http or even an invalid name. This value is reflected by req.protocol.
* The req.ip and req.ips values are populated based on the socket address and X-Forwarded-For header, starting at the first untrusted address.
Adding the capability to connect databases to Express apps is just a matter of loading an appropriate Node.js driver for the database in your app. This document briefly explains how to add and use some of the most popular Node.js modules for database systems in your Express app :
 
* Cassandra
* Couchbase
* CouchDB
* LevelDB
* MySQL
* MongoDB
* Neo4j
* Oracle
* PostgreSQL
* Redis
* SQL Server
* SQLite
* Elasticsearch
Express 5.0 is still in the beta release stage, but here is a preview of the changes that will be in the release and how to migrate your Express 4 app to Express 5.
 
To install the latest beta and to preview Express 5, enter the following command in your application root directory :
$ npm install "express@>=5.0.0-beta.1" --save