Map a range from one to another in javascript for parallax

gistfile1.txt

//map a range from one to another, protecting limits
Number.prototype.map = function(in_min, in_max, out_min, out_max) {
    var output = (this - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
    if (out_max > out_min && output < out_min)
        output = out_min;
    if (out_max > out_min && output > out_max)
        output = out_max;
    if (out_max < out_min && output < out_max)
        output = out_max;
    if (out_max < out_min && output > out_min)
        output = out_min;
    return output;
}

//Usage for parallax:
$(document).scroll(function(){
    var scrolled = window.pageYOffset;
    var topOffset = scrolled.map(0, 700, 0, -200); //for scroll position 0 to 700, offset the background by 0 to 200px
    $('.parallaxelemnt').css('background-position-y', topOffset);
});