introduction.md

Closures are a programming pattern in JavaScript which allows variables from an outer lexical scope to be used inside of a nested block of code. JavaScript supports closures transparently, and they are often used without knowing what they are.

// Top-level declarations are global-scope
const dozen = 12

{
  // Braces create a new block-scope
  // Referencing the outer variable is a closure.
  const twoDozen = dozen * 2
}

// Functions create a new function-scope and block-scope.
// Referencing the outer variable here is a closure.
function nDozen(n) {
  return dozen * n
}

Closures to save state and pass along values

Using a mutable variable declaration (like let or var) allows for some state to be preserved:

let counter = 0

// This function closure increments the counter's state in the outer lexical context.
// This way the counter can be shared between many calling contexts.
export function increment() {
  counter += 1
  return counter
}