Functions and Arrow Functions

Functions are reusable blocks of code that perform a specific task. They are the primary way to organise and reuse code in JavaScript.

Defining and Calling Functions

function greet(name) {
  return "Hello, " + name + "!";
}

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

The function keyword declares a function. Parameters are listed inside parentheses. The return statement sends a value back to the caller. If there is no return, the function returns undefined.

Function Expressions

Functions can be assigned to variables:

const double = function(n) {
  return n * 2;
};

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

Arrow Functions

Arrow functions provide a shorter syntax, introduced in ES6:

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

// Arrow function
const add = (a, b) => a + b;

// Multi-line arrow function
const multiply = (a, b) => {
  const result = a * b;
  return result;
};

// Single parameter — parentheses optional
const square = n => n * n;

// No parameters — empty parentheses required
const getRandom = () => Math.random();

When the body is a single expression, you can omit the curly braces and return keyword — the expression is returned implicitly.

Default Parameters

Provide default values for parameters that might not be supplied:

function greet(name = "stranger") {
  return "Hello, " + name + "!";
}

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

Rest Parameters

Collect an arbitrary number of arguments into an array:

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

console.log(sum(1, 2, 3));        // 6
console.log(sum(10, 20, 30, 40)); // 100

Scope

Variables declared inside a function are local to that function:

function calculate() {
  const result = 42;  // only accessible inside calculate
  return result;
}

// console.log(result);  // ReferenceError: result is not defined

Higher-Order Functions

Functions that accept other functions as arguments or return functions:

const numbers = [1, 2, 3, 4, 5];

// map — transform each element
const doubled = numbers.map(n => n * 2);    // [2, 4, 6, 8, 10]

// filter — keep elements that pass the test
const evens = numbers.filter(n => n % 2 === 0);  // [2, 4]