inViewport.js

/**
 * Credit goes to the very useful gomakethings.com
 * 
 * https://gomakethings.com/ditching-jquery/
 * 
 * Determine if an element is in the viewport
 * @param  {Node}    elem The element
 * @return {Boolean}      Returns true if element is in the viewport
 */
var isInViewport = function ( elem ) {
    var distance = elem.getBoundingClientRect();
    return (
        distance.top >= 0 &&
        distance.left >= 0 &&
        distance.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
        distance.right <= (window.innerWidth || document.documentElement.clientWidth)
    );
};

var elem = document.querySelector( '#some-element' );
isInViewport( elem ); // Boolean: returns true/false