Triggers classes on html elements based on the scroll position

It's quite simple, just add the ScrollTrigger.min.js file to your HTML page. Then call ScrollTrigger.init() when the page has loaded. Like so:

<script src="ScrollTrigger.min.js"></script>
<script>
document.addEventListener('DOMContentLoaded', function(){
  ScrollTrigger.init();
});
</script>

The init call takes 2 parameters, the bindTo and scrollIn parameters. These are optional.

The bindTo parameter is the HTML element where the triggers should be fetched from, this usually is document.body but could specify to a certain element.

The scrollIn parameter is the element to get the scroll position from, by default this is window, but it could be a scrolling div etc.

Now add the data-scroll attribute to the HTML element you want to animate:

<div data-scroll></div>

When you scroll the page, and the element is visible in the viewport, it will add the default class visible. If it's out of the viewport the invisible class is added. You can specify options in the data-scroll attribute, it can take up to 4.

<div data-scroll="150 animateIn animateOut true"></div>

The first option 150 is the offset to add to the position, in this case, it will start 150 pixels after it has been in the viewport. This is nice for tweaking animations to trigger at the right time.

The second and third options are the classes to add when visible/invisible.

The fourth and last option is a boolean, if true it'll add the element's height to the scroll offset. Now it will only add the animateIn class when the element is completely in the viewport.

You can find examples on how to use the css classes to animate in the gh-pages branch. Checkout index.html and style.css for some basic animations.

If you want to add custom animations based on the scroll position, it would be a waste to start another loop / jack the onscroll function. That's why you can attach callbacks to the ScrollTrigger's loop. This is really simple:

document.addEventListener('DOMContentLoaded', function(){
  ScrollTrigger.init();
  ScrollTrigger.attach(function(height, scrollPos){
    // i can do anything now with the height of the viewport
    // or the scrollPosition in the scrollElement. 'this' refers to
    // the ScrollTrigger object.
  });
});

Fork, check out ScrollTrigger.js and enjoy!