JS Basics

JavaScript Functions

Master JavaScript functions: declarations, expressions, arrow functions, closures, and higher-order functions.

JavaScript Functions

A JavaScript function is a block of code designed to perform a particular task. A JavaScript function is executed when "something" invokes it (calls it).

Function Declaration vs Expression

Declaration — hoisted, can be called before defined:

function name() { ... }

Expression — not hoisted, stored in variable:

const name = function() { ... }

Arrow Function — concise syntax, no own this:

const name = () => { ... }

Higher-Order Functions

Functions that take other functions as arguments or return functions. These are the foundation of functional programming in JavaScript.

Closures

A closure is a function that has access to the outer function's variables even after the outer function has returned.

Example

javascript

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

// Function expression
const multiply = function(a, b) {
  return a * b;
};

// Arrow functions
const subtract = (a, b) => a - b;
const square = x => x * x;  // single param, no parens needed

// Default parameters
const greet = (name = "World") => `Hello, ${name}!`;
console.log(greet());        // Hello, World!
console.log(greet("Alice")); // Hello, Alice!

// Rest parameters
const sum = (...nums) => nums.reduce((acc, n) => acc + n, 0);
console.log(sum(1, 2, 3, 4, 5));  // 15

// Higher-order functions
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);
const big = numbers.filter(n => n > 2);

// Closures
function makeCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    get: () => count,
  };
}
const counter = makeCounter();
counter.increment();
counter.increment();
console.log(counter.get());  // 2

Try it yourself — JAVASCRIPT