JavaScript Variables

Variables are containers for storing data (values). In JavaScript, you can use the var, let, or const keyword to declare a variable.

1. Declaring Variables

You can declare a variable using var, let, or const. The let and const keywords were introduced in ES6 (2015).

// Using var
var x = 5;
var y = 6;
var z = x + y;

// Using let
let a = 10;
let b = 20;
let c = a + b;

// Using const
const pi = 3.14;
const radius = 5;
const area = pi * radius * radius;

2. Variable Scope

The scope of a variable is the region of your program in which it is defined. JavaScript has two types of scope: global and local.

• Global Scope: A variable declared outside a function becomes global.
• Local Scope: A variable declared within a function is local to that function.

// Global variable
var globalVar = "I am a global variable";

function myFunction() {
    // Local variable
    var localVar = "I am a local variable";
    console.log(localVar);
}

myFunction();
console.log(globalVar);
// console.log(localVar); // This will cause an error

3. Variable Hoisting

In JavaScript, variable declarations are hoisted (lifted) to the top of their scope. However, the initialization is not hoisted.

console.log(hoistedVar); // Output: undefined
var hoistedVar = "This variable is hoisted";

// let and const are not hoisted
// console.log(notHoistedVar); // This will cause an error
let notHoistedVar = "This variable is not hoisted";

4. Re-declaring Variables

Variables declared with var can be re-declared. Variables declared with let and const cannot be re-declared in the same scope.

// var can be re-declared
var redeclareVar = "First declaration";
var redeclareVar = "Second declaration";

// let and const cannot be re-declared
let redeclareLet = "First declaration";
// let redeclareLet = "Second declaration"; // This will cause an error

const redeclareConst = "First declaration";
// const redeclareConst = "Second declaration"; // This will cause an error

5. Constants

Variables declared with const are read-only. They cannot be reassigned, but their properties can be changed if they are objects.

const constantVar = "I am constant";
// constantVar = "Trying to change"; // This will cause an error

const person = {
    name: "John",
    age: 30
};

// You can change properties of a constant object
person.age = 31; // This is allowed

console.log(person);

6. Best Practices

• Use let and const instead of var to avoid hoisting issues and improve code readability.
• Use const for variables that should not change.
• Declare variables at the top of their scope to avoid confusion.

By following these guidelines, you can write cleaner, more maintainable JavaScript code.