Functions

⚡Functions Beginner

Prerequisites:

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 (ES6)

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

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

// Single parameter (no parentheses needed)
const double = n => n * 2;

// No parameters
const sayHi = () => "Hi!";

Parameters & Arguments

Default Parameters

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

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

Rest Parameters

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

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

Destructuring Parameters

function createUser({ name, age, role = "user" }) {
  return { name, age, role };
}

createUser({ name: "Alice", age: 30 });

Return Values

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

// Multiple values (via object)
function getStats(numbers) {
  return {
    min: Math.min(...numbers),
    max: Math.max(...numbers),
    avg: numbers.reduce((a, b) => a + b) / numbers.length
  };
}

const { min, max, avg } = getStats([1, 2, 3, 4, 5]);

Higher-Order Functions

Functions that take or return other functions:

// Function as parameter
function repeat(fn, times) {
  for (let i = 0; i < times; i++) {
    fn(i);
  }
}

repeat(i => console.log(`Iteration ${i}`), 3);

// Function as return value
function multiplier(factor) {
  return (number) => number * factor;
}

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

Closures

Functions remember their lexical scope:

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

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

Next Steps

• Learn Arrays to work with collections of data
• Jump to Async JS to handle asynchronous operations