// Global counter
var counter = 0;

function add() {
    counter += 1;
}

// Local counter
function add() {
    var counter = 0;
    counter += 1;
}

// Nested counter
function add() {
    var counter = 0;
    function plus() {counter += 1;}
    plus();
    return counter;
}

// Closure counter
var add = (function () {
    var counter = 0;
    return function () {return counter += 1;}
})();