What is JavaScript ?
JavaScript is the world's most popular programming language.
JavaScript is the programming language of the Web.JavaScript is easy to learn.
Why Study JavaScript?
JavaScript is one of the 3 languages all web developers must learn:
- HTML to define the content of web pages
- CSS to specify the layout of web pages
- JavaScript to program the behavior of web pages
What is function?
In JavaScript, a function is a block of code designed to perform a particular task. It allows you to reuse code, organize logic, and create modular applications.
1. Declaring a Function
function greet() {
console.log("Hello, world!");
}
2. Calling a Function
greet(); // Output: Hello, world!
3. Function with Parameters
function add(a, b) {
return a + b;
}
console.log(add(5, 3)); // Output: 8
4. Function Expression
const multiply = function(x, y) {
return x * y;
};
console.log(multiply(4, 5)); // Output: 20
5.Why Functions Are Useful
Code reusability
Modular design
Easier debugging
Improved readability
JavaScript has 8 Datatypes
String
Number
Bigint
Boolean
Undefined
Null
Symbol
Object
In JavaScript, let, const, and var are used to declare variables, but they behave differently in terms of scope, hoisting, and mutability.
1. var (Old, avoid using it)
Function-scoped
Can be re-declared and updated
Hoisted (initialized as undefined)
function testVar() {
var x = 10;
if (true) {
var x = 20; // same variable
console.log(x); // 20
}
console.log(x); // 20
}
2. let (Modern, preferred for variables that change)
Block-scoped
Can be updated, but not re-declared in the same scope
Hoisted, but not initialized (accessing before declaration gives error)
function testLet() {
let y = 10;
if (true) {
let y = 20; // different variable
console.log(y); // 20
}
console.log(y); // 10
}
3. const (Modern, for constants)
Block-scoped
Cannot be updated or re-declared
Must be initialized at declaration
For objects/arrays, the reference is constant (you can still change properties)
const z = 30;
// z = 40; ❌ Error: Assignment to constant variable
const person = { name: "Alice" };
person.name = "Bob"; // ✅ Allowed: changing property
In JavaScript, the **return keyword is used inside a function to:**
Stop the function execution.
Send a value back to the place where the function was called.
Basic Syntax
function add(a, b) {
return a + b;
}
let result = add(5, 3); // result = 8
console.log(result); // Output: 8
parameters and arguments
In JavaScript, parameters and arguments are related to how functions receive data. They’re often confused, but they mean different things:
Parameters
These are placeholders in the function definition.
They define what kind of input the function can take.
function greet(name) { // 'name' is a parameter
console.log("Hello, " + name);
}
Arguments
These are the actual values you pass when calling the function.
greet("Alice"); // "Alice" is an argument
Example with Multiple Parameters and Arguments
function add(a, b) { // a and b are parameters
return a + b;
}
let sum = add(5, 3); // 5 and 3 are arguments
console.log(sum); // Output: 8