Scope and closures

An inner function can use the variables of the function around it: that's lexical scope: what you can see depends on where the code is written.

A closure is a function that keeps access to those outer variables, even after the outer function has returned.

function makeCounter() {
  let count = 0;
  return () => {
    count++;
    return count;
  };
}

const next = makeCounter();
next(); // 1
next(); // 2: count lives on inside the closure

• Each call to makeCounter() creates a fresh count, private to the function it returns.
• Closures are everywhere: every event handler or callback that uses a variable from around it is one.