sloth.js

(function(e){function t(){var t="prototype",n="scroll",r=Array[t].slice,i,s,o,u=e.setTimeout,a=[],f=function(e,t,n){this.element=e,this.threshold=t,this.callback=function(){n(e)}},l=function(){var t=a.length,r;if(t){i=e.scrollY||e.pageYOffset,s=i+e.innerHeight;while(t--)r=a[t],r.visible()&&(u(r.callback,0),a.splice(t,1))}else e.removeEventListener(n,l)};return f[t].visible=function(){var e=this.element,t=this.threshold,n=e.offsetTop-t,r=n+e.offsetHeight+t;return s>=n&&i<=r},function(t){if(t){var i=t.on,s=t.threshold!==o?t.threshold:100,u=t.callback,c;if(!i||!u)throw"elements or callback missing";if(i.length!==o){i=r.call(i),c=i.length;while(c--)a.push(new f(i[c],s,u))}else a.push(new f(i,s,u));l(),a.length&&e.addEventListener(n,l)}}}e.define&&e.define.amd?e.define("sloth",t):e.sloth=t()})(this)