JavaScript Functions & Asynchronous Programming (Notes)

 is a reusable block of code that performs a specific task. It allows us to write code once and use it multiple times.

1. Simple Function (No Arguments, No Return)

Performs an action but does not accept inputs or return a value.

function greet() {
    console.log("Hello, Student!");
}

greet(); // Output: Hello, Student!


Takes values (arguments) and uses them, but does not return anything.

function greetUser(name) {
    console.log("Hello, " + name);
}

greetUser("Alice"); // Output: Hello, Alice


3. Function without Arguments (With Return)

Returns a value that can be stored or printed.

function getPiValue() {
    return 3.14159;
}

let pi = getPiValue(); 
console.log(pi); // Output: 3.14159


function add(a, b) {
    return a + b;
}

let result = add(10, 20);
console.log(result); // Output: 30


Part 2: Understanding Arguments and Function Definitions

JavaScript does not throw an error. It simply assigns values to parameters from left to right and ignores the rest.

function show(a) {
    console.log(a);
}

show(10, 20, 30); 
// Output: 10


, which may result in unexpected output such as 

function add(a, b) {
    return a + b;
}

console.log(add(10)); 
// Output: NaN
// Explanation: 10 + undefined = NaN


Anonymous Functions and Function Expressions

const sayBye = function() {
    console.log("Goodbye!");
};

sayBye(); // Output: Goodbye!


Note: Function expressions must be defined before being called.

Part 3: IIFE (Immediately Invoked Function Expression)

An IIFE runs immediately after it is defined.

(function(name) {
    console.log("I run immediately. Hello " + name);
})("John");


Part 5: Array Methods (map, filter, reduce)

const courses = [
    { name: "Python", enrolledStudents: 1500, avgRating: 4.9, price: 6000 },
    { name: "JavaScript", enrolledStudents: 2000, avgRating: 4.5, price: 5000 },
    { name: "React", enrolledStudents: 800, avgRating: 4.2, price: 7000 },
    { name: "Data Science", enrolledStudents: 1200, avgRating: 4.8, price: 9000 }
];


Returns a new array where each element is transformed by the function.

const courseNames = courses.map(function(course) {
    return course.name;
});

console.log(courseNames);
// ["Python", "JavaScript", "React", "Data Science"]


Returns a new array containing only elements that satisfy a condition.

 Get courses with rating greater than 4.5.

const highRatedCourses = courses.filter(function(course) {
    return course.avgRating > 4.5;
});

console.log(highRatedCourses);


const totalPrice = courses.reduce(function(acc, course) {
    return acc + course.price;
}, 0);

console.log(totalPrice);
// Output: 27000


 and executes code one line at a time on a 

However, JavaScript can still perform asynchronous operations using 

Part 7: Example — Understanding setTimeout and Event Loop

console.log(1);
console.log(2);

setTimeout(function() {
    console.log("Hello");
}, 3000);

setTimeout(function() {
    console.log("HIIIIIII");
}, 5000);

console.log("Bye");
