Functions, Scope & Object Model

Lexical Scope & Closures

JavaScript uses lexical scope: where a function is written determines which variables it can access. A closure is a function retaining access to variables from its outer lexical environment even after that outer function has returned.

Scope is based on code location, not caller
A closure captures variables, not snapshots
Closures are ordinary, not exotic
Closures can be a clean encapsulation tool
Closures can retain memory

Closure keeps the environment alive

The returned function keeps access to the `count` binding after `createCounter` returns.

Private state with a closure

function createCounter() {
  let count = 0

  return {
    next() {
      count += 1
      return count
    },
    reset() {
      count = 0
    },
  }
}

const counter = createCounter()
counter.next() // 1
counter.next() // 2

`count` is not reachable except through the returned methods.

Closure uses
Use	Example	Watch out
Encapsulation	Factory with private state	Hidden state can still become hard to test
Callbacks	Event handler reads outer variable	Stale values in long-lived callbacks
Memoization	Cache closed over by function	Unbounded memory growth
Configuration	Function factory	Too many layers can obscure flow