Difference between current position and scrolled position with jQuery

I'm trying to get difference between current distance of element from the top and its next position after scrolling. In fact I'm trying to choose animation duration depending on its distance. I wrote the code below but it's not work correctly. Is there any advice to help me. Thanks a lot.

var w = window.innerWidth
 || document.documentElement.clientWidth
 || document.body.clientWidth;

 var h = window.innerHeight
 || document.documentElement.clientHeight
 || document.body.clientHeight;


var scrolingMenuTime=1500;
var hashTagActive = "";
    $(".item, .item-f").click(function (ev) {
        if(hashTagActive != this.hash) {
            ev.preventDefault();
            var next = 0;
            if ($(this.hash).offset().top > $(document).height() - $(window).height()) {
                next = $(document).height() - $(window).height();
            } else {
                next = $(this.hash).offset().top;
            }
            var currentPosition=$(this).offset().top;
            var diffPos=Math.abs(next-currentPosition);

        if (diffPos >= h && diffPos < 2*h){
            scrolingMenuTime=1700;
        }else if(diffPos >= 2*h && diffPos < 3*h){
            scrolingMenuTime=2500;
        }else if(diffPost >= 3*h && diffPos < 4*h){
            scrolingMenuTime=3500;
        }else if(diffPos >= 4*h && diffPos < 5*h){
            scrolingMenuTime=4500;
        }else if(diffPos >= 5*h){
            scrolingMenuTime=5500;}
        else{
                return false;
            }
        $('html,body').animate({
            scrollTop: next
        }, scrolingMenuTime, 'easeOutBack');
        hashTagActive = this.hash;
        location.hash = '';
    }
});

And also I have to tell that when I delete the "if and else if" section then the code works correctly. What is the wrong with "if and else if" section?