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JavaScript Variables

In JavaScript, a variable is a named storage location that holds data values. Variables are used to store and manipulate data in a program.

Declaration and Initialization

Declaration: Variables are declared using the var, let, or const keyword.

var age;    // Declaration using var
let name;   // Declaration using let
const PI = 3.14;  // Declaration using const (constant)

Initialization: Variables can be assigned values during or after declaration.

age = 25;   // Initialization
name = "Darling";  // Initialization

Variables can also be declared and initialized in a single line.

var count = 0;    // Declaration and Initialization

Variable Names

  • Variable names (identifiers) must start with a letter, underscore (_), or dollar sign ($).
  • Subsequent characters can also be digits (0-9).
  • JavaScript is case-sensitive, so age and Age are different variables.

var, let, and const

var: Declares a variable with function scope (not block scope).

var x = 10;

if (true) {
  var x = 20;  // Same variable x
}

console.log(x);  // Outputs 20

let: Declares a variable with block scope.

let y = 10;

if (true) {
  let y = 20;  // Different variable y
}

console.log(y);  // Outputs 10

const: Declares a constant variable with block scope. Its value cannot be reassigned.

const PI = 3.14;
// PI = 3.14159; // Error: Assignment to constant variable

Dynamic Typing

JavaScript is dynamically typed, meaning you can assign different data types to the same variable.

let message = "Hello, Coder!";  // String
message = 42;  // Number
message = true;  // Boolean

Best Practices

  • Use const when the value shouldn't be reassigned.
  • Use let when the value might be reassigned.
  • Avoid using var due to function scope and potential hoisting issues.

 

Summary

Understanding how to declare, initialize, and use variables is fundamental to writing JavaScript code. Variables allow you to store and manage data dynamically within your programs.

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