Understanding Functions in JavaScript

Functions in JavaScript

Functions are one of the most important building blocks in JavaScript.

They let you group reusable code into blocks, execute tasks, and return results.

A function in JavaScript is a block of code designed to perform a specific task.

It is executed when "called" (invoked).

📌 Syntax

function functionName(parameters) {
    // code to be executed
}

Defined using the function keyword.

function greet(name) {
    return `Hello, ${name}!`;
}
console.log(greet("Alice")); // Hello, Alice!

Stored in a variable.

const greet = function(name) {
    return `Hello, ${name}!`;
};
console.log(greet("Bob"));

Shorter syntax for functions.

const greet = (name) => `Hello, ${name}!`;
console.log(greet("Charlie"));

A function without a name, often used in callbacks.

setTimeout(function() {
    console.log("This runs after 2 seconds");
}, 2000);

Executes immediately after creation.

(function() {
    console.log("I run immediately!");
})();

function sum(a, b) {
    return a + b;
}
console.log(sum(5, 3)); // 8

Assign default values if no argument is passed.

function greet(name = "Guest") {
    console.log(`Hello, ${name}!`);
}
greet(); // Hello, Guest!

Functions can return values using return.

function multiply(a, b) {
    return a * b;
}
let result = multiply(4, 5);
console.log(result); // 20

Variables inside a function are local and can’t be accessed outside.

function test() {
    let x = 10;
    console.log(x); // 10
}
console.log(x); // ❌ Error: x is not defined

Functions that take other functions as arguments or return them.

function operate(a, b, operation) {
    return operation(a, b);
}
console.log(operate(5, 3, (x, y) => x + y)); // 8

A function is passed as an argument to another function.

function fetchData(callback) {
    console.log("Fetching data...");
    callback();
}
fetchData(() => console.log("Data fetched!"));

Functions in JavaScript