Node Version Manager (NVM)

Node Version Manager (NVM) is a neat little bash script that allows you to manage multiple versions of Node.js on the same box. A version manager really helps to test our applications under different versions of the related software. nvm is a tool that allows you to download and install Node.js. You don't need nvm unless you you want to keep multiple versions of Node.js installed on your system or if you'd like to upgrade your current version.

Command to check NVM version:
nvm --version.

Bash command to install NVM:
curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.26.1/install.sh | bash

How to use NVM:
1. Install Node.js
If you have not already installed Node.js on your machine, it is time to install the latest or a specific version of Node.js using NVM. Below command will install version 0.12.7 for 64-bit Windows by running the command:
$ nvm install 0.12.7 64

2. List the available Node.js versions
In order to list all the Node.js versions installed on your machine, simply run the command:
$ nvm list

3. Use a specific Node.js version
Below command used to set version by running the command:
$ nvm use 4.2.1 64

As Node.js is still a go-to solution, many versions of it are released and new versions will be released in the future. That is where testing an application with various Node.js version comes handy.

Node.JS Processing Model

Node.js processes user requests differently when compared to a traditional web server model. Node.js runs in a single process and the application code runs in a single thread and thereby needs less resources than other platforms. All the user requests to your web application will be handled by a single thread and all the I/O work or long running job is performed asynchronously for a particular request. So, this single thread doesn't have to wait for the request to complete and is free to handle the next request. When asynchronous I/O work completes then it processes the request further and sends the response.
An event loop is constantly watching for the events to be raised for an asynchronous job and executing callback function when the job completes. Internally, Node.js uses libev for the event loop which in turn uses internal C++ thread pool to provide asynchronous I/O.

This is how a single thread can handle multiple requests at once; receiving a request and either serving static/simple content or delegating it to an I/O thread from a thread pool are both very cheap and quick operations. When the thread pool thread that is doing the long-running I/O work signals to the single listener thread that the work is done, the listener thread picks up the response and sends it back to the user; this is another very cheap operation. The core idea is that the single listener thread never blocks: it only does fast, cheap processing or delegation of requests to other threads and the serving of responses to clients.

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