isElementInViewport: Get the lowest element in our viewport by a jQuery selector.

on_screen.js

/**
 * Pass in jQuery selector, for example:
 * 
 * var elements = $('.my_element');
 * console.log(isElementInViewport(elements))
 */
function isElementInViewport(elements) {
    var visibleElements = [];

    elements.each(function () {
        var $el = $(this),
            rect = $el[0].getBoundingClientRect(),
            isVisible = (
                rect.top >= 0 &&
                rect.left >= 0 &&
                rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
                rect.right <= (window.innerWidth || document.documentElement.clientWidth)
            );

        if (isVisible) {
            visibleElements.push({
                el: $el,
                bottom: rect.bottom
            });
        }
    });

    return _.first(_.where(visibleElements, {bottom: _.max(_.pluck(visibleElements, 'bottom'))}));
}

I got a reference error says: _ is not defined is that lodash or some variable you declared before ?