//main class
function Section(){
    this.currentKey;
    this.offsets = [];
    this.sectionID = ['#a','#b','#c','#d'];
}

//constructor
Section.prototype.construct = function(topOffset) {

    //sets the offsets values
    for(var u = 0; u < this.sectionID.length; u++) {
        this.offsets[u] = $(this.sectionID[u]).offset().top;
    }

    //gets the currentKey
    for (var i = 0; i < offsets.length; i++) {
        if (topOffset >= this.offsets[i] && topOffset < this.offsets[i + 1]) {
            this.currentKey = i;
        }
    }
}

//get current
Section.prototype.getCurrent = function() {
    return this.sectionID[this.currentKey];
}

//set classes
Section.prototype.setAttributes = function(){
    $('#current').Attr('id').remove();
    $(this.getCurrent).Attr('id','#current');
}

var Section = new Section();
Section.construct(topOffset,offsets);
Section.setAttributes();