Auto-Scroll to next anchor at Mouse-wheel

This works in Chrome, IE, Firefox, Opera, and Safari:

(function() {
  var delay = false;

  $(document).on('mousewheel DOMMouseScroll', function(event) {
    event.preventDefault();
    if(delay) return;

    delay = true;
    setTimeout(function(){delay = false},200)

    var wd = event.originalEvent.wheelDelta || -event.originalEvent.detail;

    var a= document.getElementsByTagName('a');
    if(wd < 0) {
      for(var i = 0 ; i < a.length ; i++) {
        var t = a[i].getClientRects()[0].top;
        if(t >= 40) break;
      }
    }
    else {
      for(var i = a.length-1 ; i >= 0 ; i--) {
        var t = a[i].getClientRects()[0].top;
        if(t < -20) break;
      }
    }
    if(i >= 0 && i < a.length) {
      $('html,body').animate({
        scrollTop: a[i].offsetTop
      });
    }
  });
})();

Fiddle at http://jsfiddle.net/t6LLybx8/728/

How it works

To monitor the mouse wheel in most browsers, use $(document).on('mousewheel'). Firefox is the oddball, and it requires $(document).on('DOMMouseScroll').

To get the direction of the mouse wheel (up or down), use event.originalEvent.wheelDelta. Again, Firefox is the oddball, and you have to use -event.originalEvent.detail.

If the direction is a negative number, you’re scrolling down the page. In that case, loop through each tag beginning with the first, until its first getClientRects() top is >= 40. (I used 40, in case the browser adds a default margin at the top of the viewport.)

If the direction is a positive number, you’re scrolling up the page. In that case, loop through each tag beginning with the last, until its first getClientRects() top is < -20. (I used -20 to ensure we move up the page.)

The delay variable prevents the mouse wheel from scrolling too quickly. The entire function is wrapped in a closure, so delay remains a private variable.