function sequence(newStart, newStep) {

    var currentCount = newStart || 0,

        step = newStep || 1,

        counter = 0;

    return function() {

        counter = currentCount;

        currentCount += step;

        return counter;

    };

}

​

​

var generator = sequence(10, 3);

console.log(generator()); // 10

console.log(generator()); // 13

console.log(generator()); // 16

console.log(generator()); // 19