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

var generator1 = idGenerator();
var generator2 = idGenerator();

console.log('generator 1 mówi: ' + generator1());
console.log('generator 2 mówi: ' + generator2());
console.log('generator 1 mówi: ' + generator1());
console.log('generator 1 mówi: ' + generator1());
console.log('generator 2 mówi: ' + generator2());
console.log('generator 1 mówi: ' + generator1());
console.log('generator 1 mówi: ' + generator1());
console.log('generator 2 mówi: ' + generator2());
console.log('generator 1 mówi: ' + generator1());