ScrollTrigger

Triggers classes on html elements based on the scroll position. It makes use of requestAnimationFrame so it doesn't jack the users scroll, that way the user / browser keeps their original scroll behaviour. Animations run when the browser is ready for it.

How to use?

It's quite simple, just add the ScrollTrigger.min.js file to your HTML page. Then construct a new ScrollTrigger instance when the page has loaded. Like so:

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

That's how simple it can be. A more advanced example would be:

document.addEventListener('DOMContentLoaded', function(){
	var trigger = new ScrollTrigger({
	  toggle: {
	    visible: 'visibleClass',
	    hidden: 'hiddenClass'
	  },
	  offset: {
	    x: 0,
	    y: 20
	  },
	  addHeight: true,
	  once: true
	}, document.body, window);
});

The init call takes 3 parameters, the first is a default set of options. The last two are 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 visible class. If it's out of the viewport the invisible class is added. Now you can define those classes in your css file and add animations to them, like so:

.invisible {
	transition: opacity 0.5s ease;
	opacity: 0.0;
}

.visible {
	transition: opacity 0.5s ease;
	opacity: 1.0;
}

Now when you scroll the page, the elements that 'come in' to the viewport fade in. A really basic example.

Options

The data-scroll attribute can take a couple of options, in contrast to v0.1, the position of the options are not strict. So you can place them anywhere inside the data-scroll tag.

Name	Type	Description	Example
toggle()	CSS Classes	These are the classes that ScrollTrigger changes, a replacement for the default visible and invisible classes. It takes 2 required parameters, the visible and invisible class.	data-scroll="toggle(.animateIn, .animateOut)" The . is not required, so data-scroll="toggle(animateIn, animateOut)" is also valid, just a bit less legible. For the default options the syntax is: { toggle: { visible: 'SomeClass', hidden: 'OtherClass' } }
offset()	Pixels	This is a custom offset you can add to the element, this is nice for tuning animations. It takes 2 required parameters, the x and y offset.	data-scroll="offset(0,50px)" for a 50px y offset.data-scroll="offset(-50px,0)" for a -50px x offset.The px value is not required, so data-scroll="offset(-50,0)" is also valid, just a bit less legible. For the default options the syntax is: { offset: { x: -50, y: 0 } }
addWidth	Boolean	This adds the offsetWidth of the element to the offset, so the visible class is only added when the element is completely in the viewport.	data-scroll="addWidth" For the default options the syntax is: { addWidth: true }
addHeight	Boolean	This adds the offsetHeight of the element to the offset, so the visible class is only added when the element is completely in the viewport.	data-scroll="addHeight" For the default options the syntax is: { addHeight: true }
centerHorizontal	Boolean	This adds the half of the offsetWidth to the offset, so the visible class is added when the element is exactly half in the viewport. Really handy for horizontal scrolling pages.	data-scroll="centerHorizontal" For the default options the syntax is: { centerHorizontal: true }
centerVertical	Boolean	This adds the half of the offsetHeight to the offset, so the visible class is added when the element is exactly half in the viewport.	data-scroll="centerVertical" For the default options the syntax is: { centerVertical: true }
once	Boolean	This makes sure the animation only runs once, if you add a callback that will also only run once.	data-scroll="once" For the default options the syntax is: { once: true }

For advanced examples on how to use the options, check out the example folder. Especially the horizontal.html file.

Callbacks

You can add callbacks to the show and hide events, e.g. when an element comes into the viewport and when it goes out. You do this by adding the data-scroll-showCallback and / or the data-scroll-hideCallback tags. We avoid using eval so the callback needs to be in the (global) window scope. If you dont want to expose your function to the window scope, you can define a custom scope by setting the trigger.callScope to your scope.

A super simple example:

<div data-scroll data-scroll-showCallback="alert('Visible')" data-scroll-hideCallback="alert('Invisible')"></div>

With a custom callScope:

var scope = {};
var trigger = new ScrollTrigger();
trigger.callScope = scope;

scope.customFunction = function(value) {
  alert(value);
};

<div data-scroll data-scroll-showCallback="customFunction('Visible')" data-scroll-hideCallback="customFunction('Invisible')"></div>

For a more advanced example check out the example folder.

What about CSS?

For more advanced CSS animations check out the example folder, specifically demo.css. This demonstrates some translate/scale animations combined with opacity animations. All the animations are done in CSS so the possibilities are (almost) endless.

JavaScript API

There are several things you can control via JavaScript.

Name	Description	Example
.bind(elements)	Binds new elements to the ScrollTrigger	.bind(document.querySelectorAll('.someClass [data-scroll]'))
.triggerFor(element)	Returns the Trigger object for the given HTMLElement	.triggerFor(document.querySelector('[data-scroll]'))
.destroy(element)	Removes the Trigger object for the given HTMLElement, leaving the DOM untouched, so the classes added by ScrollTrigger will stay put.	.destroy(document.getElementById('someTrigger'))
.destroyAll()	The same as .destroy() but than for all elements.	.destroyAll()
.reset(element)	Removes the Trigger object for the given HTMLElement, and removing all traces from ScrollTrigger, so the classes added by ScrollTrigger will be removed.	.reset(document.getElementById('someTrigger'))
.resetAll()	The same as .reset() but than for all elements.	.resetAll()
.attach(functionReference)	Attaches a callback to the ScrollTrigger loop, get's called every time the scroll position has changed. Very nice for custom animations.	Check the example below
.detach(functionReference)	Removes the callback from the ScrollTrigger loop.	Check the example below

Custom 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(){
  var trigger = new ScrollTrigger();

  var callback = function(scrollLeft, scrollTop, width, height){
    // i can do anything now with the height of the viewport
    // or the scrollPosition in the scrollElement. 'this' refers to
    // the html object that contains the data-scroll attribute.
    
    // if you are done with the callback you can detach it
    // using the ScrollTrigger.detach() method.
    trigger.detach(callback);
  };

  trigger.attach(callback);
});

Contributing

Fork, check out ScrollTrigger.js and enjoy! If you have improvements, start a new branch & create a pull request when you're all done :)

Troubleshooting

You can see really quickly if the Trigger is working by hitting 'inspect element' in the little menu that pops up when you hit the right mouse button (or ctrl + click if you're one of those oldschool Mac users). Here you can see if the visible/invisble class is toggled when you scroll past the element, is that happening? Then there is something wrong with the CSS.

If the classes don't toggle, check the JavaScript console. There might be some handy info in there.

Found an issue?

Ooh snap, well, bugs happen. Please create a new issue and mention the OS and browser (including version) that the issue is occurring on. If you are really kind, make a minimal, complete and verifiable example and upload that to codepen.

Using ScrollTrigger?

I would love to see how you use ScrollTrigger in your projects! If you are using ScrollTrigger on a live webpage, please hit me up on Twitter @erikterwan, good implementations will be referenced here.

Legacy

Looking for the old, 1.5kb minified, ScrollTrigger? Check out the legacy branch!