Functions

Overview

Functions are first-class values in JavaScript—you can pass them as arguments, return them, and assign them to variables. Choose function declarations for hoisting, arrow functions for concise callbacks and lexical this.

Syntax / Usage

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

// Expression
const multiply = function (a, b) {
  return a * b
}

// Arrow function
const square = (x) => x * x
const sum = (a, b) => {
  const total = a + b
  return total
}

// Default parameters
function greet(name = 'Guest') {
  return `Hello, ${name}`
}

// Rest parameters
function logAll(...args) {
  console.log(args)
}

Examples

Higher-order function for reusable filtering:

function filterBy(items, predicate) {
  return items.filter(predicate)
}

const adults = filterBy(users, (u) => u.age >= 18)

Immediately invoked function for a private scope:

const counter = (() => {
  let count = 0
  return () => ++count
})()

counter()  // 1
counter()  // 2

Common Mistakes

• Arrow functions as object methods when you need dynamic this
• Forgetting that return on its own line after { inserts a semicolon (ASI)
• Relying on hoisting with const/let function expressions
• Creating functions inside loops without let—classic closure bug with var

See Also

closures promises async-await destructuring