es6 closures
var makeCounter = function() { var privateCounter = 0; function changeBy(val) { privateCounter += val; } return { increment: function() { changeBy(1); }, decrement: function() { changeBy(-1); }, value: function() { return privateCounter; } } }; var counter1 = makeCounter(); var counter2 = makeCounter(); alert(counter1.value()); /* Alerts 0 */ counter1.increment(); counter1.increment(); alert(counter1.value()); /* Alerts 2 */ counter1.decrement(); alert(counter1.value()); /* Alerts 1 */ alert(counter2.value()); /* Alerts 0 */