Functions in JavaScript

Functions are first‑class values. Define them with declarations, expressions, or arrow syntax.

Declarations and expressions

function add(a, b) { return a + b } // declaration (hoisted)
const mul = function (a, b) { return a * b } // expression

Arrow functions

Concise syntax; lexical this and arguments.

const inc = n => n + 1
const sum = (a, b) => { return a + b }

Avoid arrows when you need your own this (use function declarations/expressions).

Default and rest parameters

function power(x, exp = 2) { return x ** exp }
function log(event, ...args) { console.log(event, args) }

Destructuring and params

function greet({ name = 'anon' }) { return `Hi ${name}` }

Return values

No return means undefined. Functions can return objects/arrays/other functions.

this and binding

this is dynamic in classic functions; use call/apply/bind to control it.

function show() { console.log(this.id) }
const obj = { id: 1, show }
show.call({ id: 2 }) // 2

Arrows capture this from the defining scope.

Overloads and arguments

JS has no overloads; inspect params or use object options. The arguments object is array‑like for classic functions.

IIFE (module‑like scope)

(() => { const hidden = 1 })()

Summary

• Prefer declarations for named functions; arrows for concise callbacks
• Use default/rest parameters and destructuring for ergonomic APIs
• Understand this semantics; arrows are lexical