Functions

Functions in JavaScript

Functions are reusable blocks of code that perform specific tasks. They're fundamental to organizing and structuring your code.

Function Declaration

The traditional way to define a function:

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

console.log(greet("Alice"));  // "Hello, Alice!"

// Functions can have multiple parameters
function add(a, b) {
  return a + b;
}

console.log(add(5, 3));  // 8

// Functions can have default parameters
function welcome(name = "Guest") {
  return `Welcome, ${name}!`;
}

console.log(welcome());         // "Welcome, Guest!"
console.log(welcome("Bob"));   // "Welcome, Bob!"

Function Expression

Assign a function to a variable:

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

console.log(greet("Charlie"));  // "Hello, Charlie!"

// Anonymous function (no name after 'function')
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(function(num) {
  return num * 2;
});

Arrow Functions

Modern, concise syntax (ES6+):

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

// Implicit return (no curly braces)
const add = (a, b) => a + b;

// Single parameter (no parentheses needed)
const square = x => x * x;

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

// Perfect for array methods
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(num => num * 2);
const evens = numbers.filter(num => num % 2 === 0);

Return Values

Functions can return values or perform actions:

// Returning a value
function multiply(a, b) {
  return a * b;
}

const result = multiply(4, 5);  // 20

// No return statement = returns undefined
function logMessage(msg) {
  console.log(msg);
  // No return
}

const value = logMessage("Hi");  // undefined

// Early return
function divide(a, b) {
  if (b === 0) {
    return "Cannot divide by zero";
  }
  return a / b;
}

Rest Parameters

Accept unlimited arguments:

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

console.log(sum(1, 2, 3));        // 6
console.log(sum(1, 2, 3, 4, 5));  // 15

// Mix regular and rest parameters
function introduce(greeting, ...names) {
  return `${greeting} ${names.join(", ")}!`;
}

console.log(introduce("Hello", "Alice", "Bob", "Charlie"));
// "Hello Alice, Bob, Charlie!"

Higher-Order Functions

Functions that take or return other functions:

// Function that returns a function
function multiplier(factor) {
  return function(number) {
    return number * factor;
  };
}

const double = multiplier(2);
const triple = multiplier(3);

console.log(double(5));  // 10
console.log(triple(5));  // 15

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

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