closure-ben-alman.js

function makeCounter() {
  // `i` is only accessible inside the closure created when the
  // `makeCounter` function is invoked.
  var i = 0;

  return function() {
    console.log( ++i );
  };
}

// Note that `counter` and `counter2` each have their own scoped `i`,
// because the closure containing `i` is created when the function
// `makeCounter` is invoked.

var counter = makeCounter();
counter(); // logs: 1
counter(); // logs: 2

var counter2 = makeCounter();
counter2(); // logs: 1
counter2(); // logs: 2

i; // ReferenceError: i is not defined (it only exists inside the closures)