4. Functions in JavaScript

Functions are the building blocks of JavaScript. They let you organize code, reuse logic, and create powerful abstractions. Mastering functions unlocks the full power of the language.

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4.1 Function Fundamentals

What is a Function?

A function is a reusable block of code that performs a specific task.

function greet(name) {
	return `Hello, ${name}!`;
}
console.log(greet('Alice'));

Parameters vs Arguments

• Parameter: Variable in the function definition (name above)
• Argument: Value passed to the function ('Alice' above)

Hoisting

Function declarations are hoisted (can be called before they appear):

sayHi();
function sayHi() { console.log('Hi!'); }

Function expressions are NOT hoisted:

// greet(); // Error
const greet = function() { ... };

Function Scope

Variables declared inside a function are not accessible outside.

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4.2 Parameter Types

• Required: Must be provided
• Default: Has a fallback value
• Rest: Collects extra arguments
• Destructured: Unpacks from objects/arrays

function show(a, b = 2, ...rest) {
	console.log(a, b, rest);
}
show(1); // 1 2 []
show(1, 3, 4, 5); // 1 3 [4, 5]

function printUser({name, age}) {
	console.log(name, age);
}
printUser({name: 'Sam', age: 30});

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4.3 Argument Types

• Positional: Order matters
• Spread: Expand arrays/objects
• Default: Uses fallback

function add(x, y) { return x + y; }
let nums = [2, 3];
console.log(add(...nums)); // 5

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4.4 Function Types

• Classic Function

  function sum(a, b) { return a + b; }

• Arrow Function

  const double = x => x * 2;

• Anonymous Function

  setTimeout(function() { console.log('Hi'); }, 1000);

• Higher Order Function (takes/returns functions)

  function repeat(fn, n) { for (let i = 0; i < n; i++) fn(); }

• Callback Function

  [1,2,3].forEach(function(x) { console.log(x); });

• First Class Function Functions can be assigned, passed, and returned like any value.
• Pure & Impure Functions Pure: No side effects, same output for same input.

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4.5 Advanced Concepts

Nested Functions

Functions inside functions:

function outer() {
	function inner() { console.log('Inner!'); }
	inner();
}

Scope Chain

Inner functions access variables from outer scopes.

Closures

A closure is a function that remembers its outer variables:

function makeCounter() {
	let count = 0;
	return function() {
		count++;
		return count;
	};
}
let counter = makeCounter();
console.log(counter()); // 1
console.log(counter()); // 2

IIFE (Immediately Invoked Function Expression)

(function() {
	console.log('Runs immediately!');
})();

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Next: Arrays & Objects