scrollstuff.js

// populate
$('.sections').each(function(){
  var min = $(this).offset().top
    , height = $(this).outerHeight();
  offsets.push({min: min, max: min + height});
});

// turns into this...
var offsets = [
  {min: 300, max: 600},
  {min: 600, max: 900},
  {min: 900, max: 1200}
];

// react
$(window).on('scroll', function(){
  var scrollTop = document.body.scrollTop
    , activeSectionIndex;
  $.each(offsets, function(i, obj){
    if (scrollTop >= obj.min && scrollTop < obj.max  ){
      activeSectionIndex = i;
      return false;
    }
  });
  // do something
  $('.sections').eq(activeSectionIndex).addClass('active');
});