Day2

Nodejs Training: Day 2

Installing Node.js to get started

We can download Node.js version 12, which is recommended for most users as on June 23, 2020.

To verify that you have Node.js up and running, run this:

node --version

or just

node -v

If everything is ok, it will return the version number of the currently active Node.js binary.

A very simple Node.js application

Let's start with creating a file called index.js

// index.js
console.log('Hello from Node.js');

function add(x, y) {
  return x + y;
}

let sum = add(1, 2);
console.log('Sum: ', sum);

We can now run our file using the command

node index.js

We can see that it will print the output into the terminal.

Node.js modules: require, exports, imports

Require: require are used to consume modules. It allows us to include modules in our programs. We can add built-in core Node.js modules, community-based modules (node_modules), and local modules.

Exports: The exports keyword gives us the chance to "export" our objects and methods. After that we can consume it using require in any other file.

Note: We prefix the module name with ./. That indicates that the module is a local file.

Imports: Starting with version 8.5.0+, Node.js supports ES modules natively with a feature flag and new file extension *.mjs.

Node.js Callback

A callback is a function which is called when a task is completed, thus helps in preventing any kind of blocking and a callback function allows other code to run in the meantime. Callback is called when task get completed and is asynchronous equivalent for a function. Using Callback concept, Node.js can process a large number of requests without waiting for any function to return the result which makes Node.js highly scalable. For example: In Node.js, when a function start reading file, it returns the control to execution environment immediately so that the next instruction can be executed. Once file I/O gets completed, callback function will get called to avoid blocking or wait for File I/O.

What Is NPM?

NPM is the package manager used by Node.js applications - we can find a ton of modules here, so that we don't have to reinvent the wheel. There are two primary interfaces you will interact with - the NPM website and the NPM command line toolkit.

The NPM website

• https://www.npmjs.com/

The Command Line Interface (CLI)

• The NPM is bundled with the Node.js binary, so we don't have to install it. To verify that we have NPM up and running, run this:

  npm -v

  If everything is ok, it will return the version number of the installed npm.

Using NPM

1. Using npm init to Initialize a Project

   The npm init command is a step-by-step tool to scaffold out our project. It will prompt us for input for a few aspects of the project in a order.

   npm init

2. Install Modules with npm install

   Installing modules from npm is one of the most basic things we should learn to do when getting started with npm. We can do it like:

   npm install <module>

   In the above command, you'd replace <module> with the name of the module we want to install. For example, if you want to install Lodash we could run the following command:

   npm install lodash

3. Install modules and save them to your package.json as a dependency

   When we're running npm install to install a module, we can add the optional flag --save to the command. This flag will add the module as a dependency of our project to the project's package.json as an entry in dependencies.

   npm install <module> --save

4. Install Modules and Save Them to Your package.json as a Developer dependency

   When we're running npm install to install a module, we can add the optional flag --save-dev to the command. This flag will add the module as a development dependency of our project to the project's package.json as an entry in devDependencies.

   npm install <module> --save-dev

5. Install Modules Globally on your System

   To install a module from npm globally, we'll simply need to use the --global or -g flag when running the install command to have the module install globally, rather than locally (to the current directory).

   npm install <module> -g