JavaScript Essentials: Type, Scope, Functions, and More

What is JavaScript?

JavaScript is a scripting language that runs in web browsers and is used to add dynamic functionality to web pages.
Since ES6 (ECMAScript 2015), JavaScript has introduced many new features, making programming more powerful and efficient.

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Key Features

• Dynamic typing: No need to declare variable types.
• Prototype-based object-oriented programming: Objects can be created and inherited without using classes.
• Functional programming: Functions are treated as first-class objects.
• Event-driven programming: Reacts to user interactions.

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Running JavaScript with Node.js

To execute a JavaScript file in a Node.js environment, use the following command in the terminal:

node index.js

• Node.js must be installed.
• The index.js file must exist in the current directory.

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Basic Syntax

Variable Declaration

Keyword	Scope	Redeclaration	Reassignment
var	Function or global	Yes	Yes
let	Block	No	Yes
const	Block	No	No

var x = 10;
var x = 20; // Allowed
x = 30;     // Allowed

let y = 10;
y = 30;     // Allowed

const z = 10;
// z = 20;  // Error: Cannot reassign

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Semicolons and Comments

• Semicolons (;): Optional but recommended for clarity.
• Comments:

// Single-line comment
let x = 10; // Variable declaration

/*
Multi-line comment
Declaring and initializing variable y.
*/
let y = 20;

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Logging Output

console.log("Hello, World!"); // Outputs a string

let a = 5;
let b = 10;
console.log(a + b); // 15

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Scope and First-Class Functions

Scope

JavaScript variables and functions have different levels of accessibility:

• Global Scope: Accessible anywhere in the code.
• Function Scope: Available only within the function.
• Block Scope: Restricted to {} when using let and const.

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First-Class Functions

Functions in JavaScript are first-class objects:

const sayHello = function() {
    console.log("Hello!");
};

function executeFunction(func) {
    func();
}

executeFunction(sayHello); // "Hello!"

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Hoisting

Hoisting refers to JavaScript moving variable and function declarations to the top of their scope.

console.log(x); // undefined
var x = 5;

console.log(y); // Error: Cannot access 'y' before initialization
let y = 10;

function sayHello() {
    console.log("Hello!");
}
sayHello(); // "Hello!"

• var variables are hoisted and initialized as undefined.
• let and const are hoisted but not initialized (Temporal Dead Zone).

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Data Types & Operators

Data Types

Primitive Types

let num = 42;       // Number
let name = "Alice"; // String
let isActive = true; // Boolean
let value;          // undefined
let empty = null;   // Null
let sym = Symbol("id"); // Symbol
let bigNumber = 123456789012345678901234567890n; // BigInt

Reference Types

let person = { name: "Alice", age: 25 }; // Object
let numbers = [1, 2, 3]; // Array

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Operators

Arithmetic Operators

console.log(5 + 3); // 8
console.log(10 - 4); // 6
console.log(6 * 7); // 42
console.log(10 / 2); // 5
console.log(10 % 3); // 1
console.log(2 ** 3); // 8

Comparison Operators

console.log(5 == "5");  // true (loose equality)
console.log(5 === "5"); // false (strict equality)
console.log(5 != "5");  // false (loose inequality)
console.log(5 !== "5"); // true (strict inequality)
console.log(10 > 5);    // true
console.log(5 <= 5);    // true

Logical Operators

console.log(true && false); // false (AND)
console.log(true || false); // true (OR)
console.log(!true);         // false (NOT)

Ternary Operator

let age = 18;
let status = (age >= 18) ? "Adult" : "Minor";
console.log(status); // Adult

Type Conversion Operators

console.log(typeof 42); // "number"
console.log(typeof "hello"); // "string"

let arr = [];
console.log(arr instanceof Array); // true

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Conclusion

JavaScript is a versatile language used in both front-end and back-end development.
Understanding the fundamentals is essential for writing efficient and maintainable code.

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