Modules that Node.js ships with. Also referred to as core modules.

Import modules before you can use it.

• path
• events
• fs
• stream
• http

Path module

The path module provides utilities for working with file and directory path.

js
const path = require("node:path");

console.log(__filename); // path to current file
console.log(__dirname); // path to cwd

path.basename

js
const path = require("node:path");

console.log(path.basename(__filename)); //index.js
console.log(path.basename(__dirname)); // Node JS

path.extname

js
const path = require("node:path");

console.log(path.extname(__filename)); // .js
console.log(path.extname(__dirname)); // 

path.parse

js
const path = require("node:path");

console.log(path.parse(__filename));
/* Output:
{
    root: '/',
    dir: '/path_to_dir',
    base: 'index.js',
    ext: '.js',
    name: 'index'
}
*/

path.format

js
const path = require("node:path");

console.log(path.format(path.parse(__filename)));
// /path_to_file

path.isAbsolute

js
const path = require("node:path");

console.log(path.isAbsolute(__filename)); // true

path.join

js
const path = require("node:path");

console.log(path.join("folder1", "folder2", "index.html"));
// folder1/folder2/index.html
console.log(path.join("/folder1", "folder2", "index.html"));
// /folder1/folder2/index.html
console.log(path.join("/folder1", "//folder2", "index.html"));
// /folder1/folder2/index.html
console.log(path.join("/folder1", "//folder2", "../index.html"));
// /folder1/index.html
console.log(path.join(__dirname, "data.json"));
// /path_to_cwd/NodeJS/data.json

path.resolve

js
const path = require("node:path");

console.log(path.resolve("folder1", "folder2", "index.html"));
// /path_to_cwd/folder1/folder2/index.html
console.log(path.resolve("/folder1", "folder2", "index.html"));
// /folder1/folder2/index.html
console.log(path.resolve("/folder1", "//folder2", "index.html"));
// /folder2/index.html
console.log(path.resolve("/folder1", "//folder2", "../index.html"));
// /index.html
console.log(path.resolve(__dirname, "data.json"));
// /path_to_cwd/NodeJS/data.json

Callbacks:
In JS, functions are first class objects.

A function can be passed as an argument to a function.

A function can also be returned as values from other functions.

js
function greet(name){
    console.log(`Hello ${name}`);
}

function hof(callback){
    const name = 'Vishwas';
    callback(name);
}

hof(greet); // Hello Vishwas

Types of Callbacks:

1. Synchronous callbacks: A callback which is executed immediately is called a synchronous callback.

js
// Synchronous callbacks example
let numbers = [1, 2, 4, 7, 3, 5, 6]
numbers.sort((a, b) => a-b);
numbers.filter(n => n % 2 === 0);
numbers.map(n => n/2);

2. Asynchronous callbacks: A callback that is often used to continue or resume code execution after an asynchronous operation has completed. Asynchronous callbacks are used to delay the execution of a function until a particular time or event has occurred.
   
   In Node.js have an asynchronous nature to prevent blocking of execution.
   Ex: reading data from a file, fetching data from a database or handling a network request.

js
// Example in browser
function callback() {
    document.getElementById("demo").innerHTML = "Hello World";
}
document.getElementById("btn").addEventListener("click", callback);