JavaScript Functions

JavaScript Fundamentals

JavaScript Functions

Beginner Lesson 4 of 10

JavaScript Functions

Functions are reusable blocks of code that perform specific tasks.

Function Declaration

function greet(name) {
  return `Hello, ${name}!`;
}

greet("Alice"); // "Hello, Alice!"

Function Expression

const greet = function(name) {
  return `Hello, ${name}!`;
};

Arrow Functions

// Full syntax
const greet = (name) => {
  return `Hello, ${name}!`;
};

// Short syntax (implicit return)
const greet = name => `Hello, ${name}!`;

// Multiple parameters
const add = (a, b) => a + b;

Parameters

Default Parameters

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

greet();        // "Hello, World!"
greet("Alice"); // "Hello, Alice!"

Rest Parameters

function sum(...numbers) {
  return numbers.reduce((a, b) => a + b, 0);
}

sum(1, 2, 3, 4); // 10

Closures

function createCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    decrement: () => --count,
    getCount: () => count
  };
}

const counter = createCounter();
counter.increment(); // 1
counter.increment(); // 2
counter.getCount();  // 2

Higher-Order Functions

// Function that takes a function as argument
function repeat(n, action) {
  for (let i = 0; i < n; i++) {
    action(i);
  }
}

repeat(3, console.log); // 0, 1, 2

// Function that returns a function
function multiplier(factor) {
  return num => num * factor;
}

const double = multiplier(2);
double(5); // 10

Next Steps

Continue to Objects →