JavaScript Variables and Expressions

What are the Variables

Variables are used to store and manage data in JavaScript.
They act as containers for values, and their contents can change during program execution.
These data values can be numbers, text, booleans, or more complex data types such as objects, arrays, etc.

Declaring Variables in JavaScript

Declare variables using var, let, or const.
var variables are function-scoped, which means that they can be accessed from anywhere within the function in which they are declared.
Historically, this was the primary way to declare variables, but it has some issues (like hoisting and global scope). We recommend avoiding it.
let: Introduced later, let is the preferred way to declare variables. It provides block-level scoping and avoids the pitfalls of var.
const: Used for constants, const declares a variable that cannot be changed after its initial assignment.

Let's declare a variable using let

let message; // Declaring a variable without assigning a value

In the above example, message is a variable, and its default value is undefined.

Once declared, we can assign a value to a variable using the = operator.

let greeting;
greeting = "Hello, JavaScript!"; // Assigning a string value

Alternatively, we can declare and initialize a variable in a single line

let name = "John"; // Assigned a string value
let age = 30; // Assigned a numeric value
let isStudent = true; // Assigned a boolean value

You can declare multiple variables in a single line.

let firstName = "John", lastName = "Doe", score = 85;

You can copy the value of one variable to another.

let originalValue = 100;
let copiedValue = originalValue;

JavaScript allows multiple spaces and line breaks when declaring variables.

let   city   =   "New York",
population = 8_500_000;

Naming Conventions

Variable names must begin with a letter, underscore (_), or dollar sign ($).
They cannot start with a digit (0-9).
Variable names are case-sensitive (e.g., msg, MSG, Msg, and mSg are different variables).
Avoid using reserved keywords like var, function, or return.

Valid Variable Names

myVariable
_myVariable
$myVariable
ageInDays
productPrice
totalNumberOfCustomers

Invalid Variable Names

1myVariable
my-variable
my variable
const
function
var

Dynamic Typing

JavaScript is loosely typed, meaning you don't need to specify a variable's data type explicitly. It adapts based on the assigned value. That means it can hold different types of values.

let numberVar = 42;       // Number
let stringVar = "Hello"; // String
let boolVar = true;      // Boolean
let arrayVar = [1, 2, 3]; // Array
let objVar = { key: "value" }; // Object

Updating Variables

Use the assignment operator (=) to update a variable's value.

let count = 5;
count = count + 1; // Increment by 1 now count value is 6

Expressions

Expressions are combinations of values, operators, and variables.
They are used to perform operations and produce results.

let total = 2 * (5 + 3); // total equals 16

String Concatenation

Strings can be concatenated using the + operator.

let firstName = "Jack";
let lastName = "Salvatore";
let fullName = firstName + " " + lastName; // fullName equals "Jack Salvatore"

Template Literals

Template literals (enclosed in backticks) allow for easy string interpolation.

let name = "Alice";
let greeting = `Hello, ${name}!`; // greeting equals "Hello, Alice!"

A Complete JavaScript Variables Examples

// Declare a variable and assign it a value
var myName = "Jack Salvatore";

// Declare a variable without assigning it a value
let myAge;

// Assign a value to the variable `myAge`
myAge = 30;

// Declare a constant and assign it a value
const MY_FAVORITE_NUMBER = 7;

// Use variables in an expression
var myTotal = myAge + MY_FAVORITE_NUMBER;

// Use an expression in a variable assignment
let myAverage = (myAge + MY_FAVORITE_NUMBER) / 2;

// Log the values of the variables to the console
console.log(myName); // Outputs "Jack Salvatore"
console.log(myAge); // Outputs 30
console.log(MY_FAVORITE_NUMBER); // Outputs 7
console.log(myTotal); // Outputs 37

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