Functions

Functions are reusable blocks of code that perform a specific task. They are one of the most important concepts in JavaScript — almost everything you build will be organised into functions.

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Function Declarations

The classic way to define a function:

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

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

Key Characteristics

• Hoisted — you can call the function before its declaration in the code.
• The return statement sends a value back to the caller.
• If no return is specified, the function returns undefined.

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Function Expressions

A function assigned to a variable:

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

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

Function expressions are not hoisted — you must define them before you use them.

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Arrow Functions

Introduced in ES6, arrow functions provide a shorter syntax:

// Standard arrow function
const greet = (name) => {
  return "Hello, " + name + "!";
};

// Concise body (implicit return for single expressions)
const double = (x) => x * 2;

// No parentheses needed for a single parameter
const square = x => x * x;

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

Arrow Functions vs Regular Functions

Feature	Regular Function	Arrow Function
this binding	Dynamic (depends on how it is called)	Lexical (inherits from surrounding scope)
arguments object	Available	Not available (use rest parameters)
Hoisting	Yes (declarations)	No
Use as constructor	Yes (new MyFunc())	No
Best for	Methods, constructors	Callbacks, functional-style code

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Parameters and Arguments

Default Parameters

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

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

Rest Parameters

Collect any number of arguments into an array:

function sum(...numbers) {
  let total = 0;
  for (const n of numbers) {
    total += n;
  }
  return total;
}

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

Destructuring Parameters

function printUser({ name, age }) {
  console.log(name + " is " + age + " years old.");
}

printUser({ name: "Alice", age: 30 });
// "Alice is 30 years old."

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Scope

Scope determines where variables are accessible.

Global Scope

Variables declared outside any function or block are globally accessible:

const appName = "MyApp"; // global

function showApp() {
  console.log(appName); // accessible here
}

Function Scope