Introduction to Node.js

For this lesson, you must have Node.js installed. We will write our codes in a file called intro.js

console.log("Hello Node.js!") 

Now we can run the javascript by typing the following in the terminal:

$ node intro.js 

Hello Node.js!

Next, let's define a variable and print that out.

let message = "Hello world from the Node.js"; 

console.log(message); 

Note that you can omit the .js from the filename when running node command:

node intro 

Hello world from the Node.js

Here the console is a global variable and it is available to us without importing or requiring any class. There are several other global variables like __dirname or `__filename

console.log(__dirname); 
console.log(__filename); 

/Users/Pranab/Documents/github/js-learning/src/node
/Users/Pranab/Documents/github/js-learning/src/node/intro.js

We can import external modules by using require. Let's say we import the path module which ships with the Node.js ans load that into a variable called path. Then we try to extract just the filename using a path method.

const  path = require('path'); 
console.log(`The file name is : ${path.basename(__filename)}`); 

The file name is : intro.js

Note that for template strings back tics are used instead of single/double quotes.

Another important global object is process.

console.log(process.pid); 
console.log(process.versions.node); 

89046
12.16.2

process.argv to pass arguments:

console.log(process.argv); 

[
  '/usr/local/bin/node',
  '/Users/Pranab/Documents/github/js-learning/src/node/intro'
]

So when we run our program by typing node intro, we are simply passing a array. We can pass more variables:

node intro hello "pranab das" 

[
  '/usr/local/bin/node',
  '/Users/Pranab/Documents/github/js-learning/src/node/intro',
  'hello',
  'pranab das'
] 

Now we can create flags to pass more arguments to any command.

node intro --user "pranab das" --greeting hello

[
  '/usr/local/bin/node',
  '/Users/Pranab/Documents/github/js-learning/src/node/intro',
  '--user',
  'pranab das',
  '--greeting',
  'hello'
]

const grab = flag => {
    let indexAfterFlag = process.argv.indexOf(flag) + 1; 
    return process.argv[indexAfterFlag];
} 

const greeting = grab("--greeting"); 
const user = grab("--user"); 

console.log(greeting, user); 

node intro --user "pranab das" --greeting hello

hello pranab das