JavaScript Functions #

Functions are one of the fundamental building blocks in JavaScript. A function is a reusable block of code that performs a specific task.

Defining Functions #

There are several ways to define a function in JavaScript.

Function Declaration #

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

Function Expression #

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

Arrow Function #

Arrow functions provide a more concise syntax for writing functions.

const greet = (name) => {
 return `Hello, ${name}!`;
};
console.log(greet("World"));
// For single expression functions, you can omit the braces and return statement
const add = (a, b) => a + b;
console.log(add(2, 3)); // 5

Function Parameters #

JavaScript functions can accept parameters. You can also define default and rest parameters.

// Default parameters
function greet(name = "Guest") {
 console.log(`Hello, ${name}!`);
}
greet(); // "Hello, Guest!"
greet("Alice"); // "Hello, Alice!"
// Rest parameters
function sum(...numbers) {
 return numbers.reduce((acc, current) => acc + current, 0);
}
console.log(sum(1, 2, 3)); // 6

Higher-Order Functions #

Functions that operate on other functions, either by taking them as arguments or by returning them, are called higher-order functions.

function withLogging(fn) {
 return function(...args) {
 console.log(`Calling function ${fn.name}`);
 return fn(...args);
 };
}
function add(a, b) {
 return a + b;
}
const loggedAdd = withLogging(add);
console.log(loggedAdd(3, 4));