The Magic of Lexical Environment

JavaScript is a language with functional memory. Functions " remember " where they were created and what variables were available to them.

What is a closure?

It is a function's ability to remember its lexical scope even after the outer function has finished executing.

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

let counter = createCounter();
alert(counter()); // 0
alert(counter()); // 1

How does it work?

Inside every function, there is a hidden property [[Environment]] that stores a reference to its birthplace.