'use strict'
var sequence = function(start, step) {
    if (start) {
        var currentValue = start;
    } else
    {
        var currentValue = 0;
    }
    if(!step) {
        step = 1;
    }
    return function() {
        currentValue = currentValue + step ;
        return currentValue;
    }
}

var generator = sequence(2, 4);
console.log(generator());