function sequence(newStart, newStep) {

    var currentCount = newStart ? newStart : 0;

    return function() {

        currentCount = newStep ? currentCount + newStep : ++currentCount;

        return currentCount;

    }

}

​

​

var generator = sequence(10, 3);

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

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

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

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