Hector and Coco

function hector() {

  let spirit = "Remember Me"
	
  return function() {
    console.log(spirit);
  }
}

let coco = hector();
// 💀 Hector is dead.

coco();

coco = null;
// 🗑️ Hector's spirit will be collected.

Closure examples

function delayLog(message, time) {
  
  setTimeout(function() {
    console.log(message);
  }, time);

}

delayLog("Remember this!", 1000);

function compose (f, g) { 
  return function (x) {
    return f(g(x));
  }
}

const addOne = x => x + 1;
const double = x => x * 2;

const addOneThenDouble = compose(double, addOne);

console.log(
  addOneThenDouble(3)
);

// 8

Lexical scope

const globalVar = "global";

function outer(outerParam) {
  const outerVar = "outerVar";

  function inner(innerParam) {
    const innerVar = "innerVar";
        
    console.log(globalVar);     // ✅ global
    console.log(outerVar);      // ✅ outerVar
    console.log(outerParam);    // ✅ outerParam
    console.log(innerVar);      // ✅ innerVar
    console.log(innerParam);    // ✅ innerParam
  }
    
  return inner;
}

const innerOut = outer("outerParam");

innerOut("innerParam");

console.log(typeof outerVar);   // 🚫 undefined
console.log(typeof innerVar);   // 🚫 undefined

Counter with class

class Counter {
  constructor(addValue) {
    this.count = 0;
    this.addValue = addValue;
  }

  increment() {
    this.count += this.addValue;
    console.log(this.count);
  }
}

const counterBy1 = new Counter(1);
const counterBy3 = new Counter(3);

// Repeat below
counterBy1.increment();
counterBy3.increment();

Counter with closure

function createCounter(addValue) {
  let count = 0;

  return function() {
    count += addValue;
    console.log(count);
  }
}

const counterBy1 = createCounter(1);
const counterBy3 = createCounter(3);

// Repeat below
counterBy1();
counterBy3();

# Python version
def create_counter(add_value):
  count = 0

  def counter():
    nonlocal count
    count += add_value
    print(count)

  return counter

counter_by_1 = create_counter(1)
counter_by_3 = create_counter(3)

# Repeat below
counter_by_1()
counter_by_3()