Calculate which element is the current one.

currentElem.js

/*
TODO:
+ factor in proximity to page center
+ factor in area instead of just height
*/

function currentElem(selector){
    var sections = document.querySelectorAll(selector);
    
    var contenders = [];
    
    for (var s = 0; s < sections.length; s++) {
        var elem = sections[s];
        var item = {
            elem: elem,
            onScreen: (function(elem){
                // elem array
                var eArr = [];
                for (var e = elem.offsetTop; e <= elem.offsetTop + elem.offsetHeight; e++) {
                    eArr.push(e);
                }

                // screen array
                var sArr = [];
                for (var s = window.scrollY; s <= window.scrollY + window.innerHeight; s++) {
                    sArr.push(s);
                }

                // calculate overlap
                var overlap = [];

                eArr.forEach(function(v){
                    if (sArr.indexOf(v) !== -1) {
                        overlap.push(v)
                    }
                });

                return (overlap[overlap.length - 1] - overlap[0])
            })(elem)
        };
        contenders.push(item);
    }
    
    contenders = contenders.filter(function(item){
        return isFinite(item.onScreen);
    });
    
    contenders.sort(function(a, b){
        if (a.onScreen > b.onScreen) {
            return -1;
        } else if (b.onScreen > a.onScreen) {
            return 1;
        }
    });
    
    if (contenders[0]) {
        return contenders[0].elem.getAttribute("id");
    }
    return false;
}