counter.js

//Verison 0
'use strict';

var counterFactory = function counterFactory() {
  return function() {};
};

var firstCounter = counterFactory();
// firstCounter contains a reference to
// a new instance of the minimal function
var secondCounter = counterFactory();
// secondCounter contains a reference to
// a new instance of the minimal function
// it is a different object than the one
// stored in firstCounter

console.log('types match:',
  typeof firstCounter === typeof secondCounter);
console.log('objects match:',
  firstCounter === secondCounter);

// Version 1
'use strict';

var counterFactory = function counterFactory() {
  return function() {};
};

var firstCounter = counterFactory();
// firstCounter contains a reference to
// a new instance of the minimal function
var secondCounter = counterFactory();
// secondCounter contains a reference to
// a new instance of the minimal function
// it is a different object than the one
// stored in firstCounter

console.log('types match:',
  typeof firstCounter === typeof secondCounter);
console.log('objects match:',
  firstCounter === secondCounter);

//Version 2
'use strict';

var counterFactory = function counterFactory() {
  var counter = 1;
  return function() {
    return counter++;
  };
};

var firstCounter = counterFactory();
// firstCounter contains a reference to
// a new instance of a function that
// has access to a memory location
// it calls counter that contains the number 1
var secondCounter = counterFactory();
// secondCounter contains a reference to
// a new instance of a function that
// has access to a memory location
// it calls counter that contains the number 1
// it is a different object than the one
// stored in firstCounter and what it refers to
// with the name counter is a different memory
// location than the one firstCounter accesses

console.log('firstCounter:', firstCounter());
console.log('firstCounter:', firstCounter());
console.log('firstCounter:', firstCounter());
console.log('secondCounter:', secondCounter());
console.log('secondCounter:', secondCounter());
console.log('firstCounter:', firstCounter());

//Version 3
'use strict';

var counterFactory = function counterFactory(initial) {
  var counter = initial || 1;
  return function() {
    return counter++;
  };
};

var firstCounter = counterFactory();
// firstCounter contains a reference to
// a new instance of a function that
// has access to a memory location
// it calls counter that contains the number 1
var secondCounter = counterFactory(100);
// secondCounter contains a reference to
// a new instance of a function that
// has access to a memory location
// it calls counter that contains the number 1
// it is a different object than the one
// stored in firstCounter and what it refers to
// with the name counter is a different memory
// location than the one firstCounter accesses

console.log('firstCounter:', firstCounter());
console.log('firstCounter:', firstCounter());
console.log('firstCounter:', firstCounter());
console.log('secondCounter:', secondCounter());
console.log('secondCounter:', secondCounter());
console.log('firstCounter:', firstCounter());