test.js

function func() {
    var i = 0;
    return function () {
        
        return i++;
    }
}

var counter = func();
console.log(counter()); // 0
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3
console.log(counter()); // 4
counter = func();
console.log(counter()); // 0