Web Technologies - Sy BSc CS: Programming Fundamentals of JavaScript

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Programming Fundamentals of JavaScript

Variables:

Definition and Role:

Variables are fundamental components of any programming language, including JavaScript. They act as containers to store data values, such as numbers, strings, objects, and more, during the execution of a script. Variables play a vital role in storing information that can be accessed and manipulated throughout the program's lifecycle.

Keywords: var, let, and const:

JavaScript offers three keywords for declaring variables: `var`, `let`, and `const`.

- `var`: Before the introduction of `let` and `const` in ECMAScript 6 (ES6), `var` was the primary keyword used to declare variables. However, `var` has some scoping issues and is not recommended for modern JavaScript development.

- `let`: Introduced in ES6, `let` is block-scoped, meaning it is limited in scope to the block (enclosed by curly braces) where it is declared. It allows the variable's value to be reassigned.

- `const`: Also introduced in ES6, `const` declares a constant variable whose value cannot be changed after initialization. Like `let`, it is block-scoped.

Examples of Variable Declarations and Assignments:

Let's explore some examples of variable declarations and assignments using the different keywords:

// Using 'var':
var age = 25; // Declaring a variable 'age' and assigning a numeric value 25
var name = "John"; // Declaring a variable 'name' and assigning a string value

// Using 'let':
let count = 10; // Declaring a variable 'count' and assigning a numeric value 10
count = count + 5; // Reassigning 'count' with the new value 15

// Using 'const':
const PI = 3.14; // Declaring a constant variable 'PI' with the value 3.14

Operators:

Arithmetic Operators:

Arithmetic operators perform mathematical calculations on numeric values. Common arithmetic operators include addition (+), subtraction (-), multiplication (*), division (/), and the modulus operator (%).

var num1 = 10;
var num2 = 5;

var sum = num1 + num2; // Addition: 10 + 5 = 15
var difference = num1 - num2; // Subtraction: 10 - 5 = 5
var product = num1 * num2; // Multiplication: 10 * 5 = 50
var quotient = num1 / num2; // Division: 10 / 5 = 2
var remainder = num1 % num2; // Modulus: 10 % 5 = 0

Assignment Operators:

Assignment operators are used to assign values to variables. The most basic assignment operator is `=`.

var x = 10;
var y = 5;

x += y; // Equivalent to x = x + y, so x becomes 15

Comparison Operators:

Comparison operators compare two values and return a Boolean value (true or false) based on the comparison result.

var a = 10;
var b = 5;

var isEqual = a === b; // Equality check: false
var isNotEqual = a !== b; // Inequality check: true
var isGreater = a > b; // Greater than check: true
var isLess = a < b; // Less than check: false
var isGreaterOrEqual = a >= b; // Greater than or equal check: true
var isLessOrEqual = a <= b; // Less than or equal check: false

Logical Operators:

Logical operators are used to combine and manipulate Boolean values. The common logical operators are `&&` (logical AND), `||` (logical OR), and `!` (logical NOT).

var p = true;
var q = false;

var result1 = p && q; // Logical AND: false
var result2 = p || q; // Logical OR: true
var result3 = !p; // Logical NOT: false

Control Flow Statements:

if-else Statements:

The `if` statement allows you to execute a block of code if a certain condition is true. You can also use `else` to specify a block of code to be executed when the condition is false.

var age = 18;

if (age >= 18) {
  console.log("You are an adult.");
} else {
  console.log("You are a minor.");
}

switch Statement:

The `switch` statement allows you to perform different actions based on different conditions.

var day = "Monday";
var message;

switch (day) {
  case "Monday":
    message = "It's Monday. Back to work!";
    break;
  case "Friday":
    message = "It's Friday! Weekend is coming.";
    break;
  default:
    message = "Enjoy your day!";
}

for Loop:

The `for` loop is used to execute a block of code multiple times with a specified condition.

for (var i = 1; i <= 5; i++) {
  console.log("Count: " + i);
}

while Loop:

The `while` loop continues to execute a block of code as long as the specified condition is true.

var count = 1;

while (count <= 5) {
  console.log("Count: " + count);
  count++;
}

do-while Loop:

The `do-while` loop is similar to the `while` loop, but it always executes the block of code at least once before checking the condition.

var count = 1;

do {
  console.log("Count: " + count);
  count++;
} while (count <= 5);

Conclusion:

Understanding variables, operators, and control flow statements forms the bedrock of JavaScript programming. By mastering these fundamentals, you gain the ability to store and manipulate data, perform various operations, and control the flow of your JavaScript code, laying the groundwork for creating dynamic and interactive web applications.