Functions Deep Dive

Functions are first-class citizens in JavaScript — they can be assigned to variables, passed as arguments, and returned from other functions. This is the foundation of functional programming patterns heavily tested in interviews.

Function Declaration vs Expression

// Declaration — hoisted fully
function add(a, b) { return a + b; }

// Expression — hoisted as undefined
const subtract = function(a, b) { return a - b; };

// Arrow function (ES6)
const multiply = (a, b) => a * b;

Arrow Functions vs Regular Functions

Key difference: Arrow functions do NOT have their own this, arguments, or prototype.

const obj = {
  value: 42,
  regular: function() { return this.value; }, // 42
  arrow: () => this.value // undefined (or window.value)
};

IIFE (Immediately Invoked Function Expression)

(function() {
  const secret = 'hidden';
  console.log('Runs immediately!');
})();
// secret is not accessible here

Used to create private scope, avoiding global pollution — classic interview topic.

Higher-Order Functions

A function that takes another function as argument or returns a function.

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);     // [2,4,6,8,10]
const evens = numbers.filter(n => n % 2 === 0); // [2,4]
const sum = numbers.reduce((acc, n) => acc + n, 0); // 15