Closures

A closure is a function that keeps access to the variables of the scope where it was defined, even after that scope has finished running.

Basic example

counter.js

function createCounter() {
  let count = 0;

  return function increment() {
    count += 1;
    return count;
  };
}

const counter = createCounter();
counter(); // 1
counter(); // 2

Tip

count is private: the only way to change it is through increment.

Common pitfall: var in loops

for (var i = 0; i < 3; i++) {
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}

With var it prints 3 3 3. With let each iteration gets its own binding, so it prints 0 1 2.

Same idea in other languages

TypeScript

function createCounter(): () => number {
  let count = 0;
  return () => ++count;
}

Python

def create_counter():
    count = 0

    def increment():
        nonlocal count
        count += 1
        return count

    return increment

Terminal

Run it

node counter.js