JS function to update HTML body in place (via https://news.ycombinator.com/item?id=26696455)

update-html-body-in-place.js

function update_in_place(old_root, new_root) {
    function are_compatible(lhs, rhs) {
        return lhs.nodeType === rhs.nodeType && lhs.tagName === rhs.tagName && lhs.id === rhs.id
    }
    if (old_root.nodeType === 3 && new_root.nodeType === 3) {
        old_root.textContent = new_root.textContent;
        return old_root
    }
    if (!are_compatible(old_root, new_root) || old_root.nodeType !== 1) {
        old_root.parentElement.replaceChild(new_root, old_root);
        return new_root
    }
    for (var i = 0; i < new_root.attributes.length; i++) {
        var a = new_root.attributes[i];
        if (a.specified && old_root.getAttribute(a.name) !== a.value) {
            old_root.setAttribute(a.name, a.value)
        }
    }
    for (var i = 0; i < old_root.attributes.length; i++) {
        var a = old_root.attributes[i];
        if (a.specified && !new_root.hasAttribute(a.name)) {
            old_root.removeAttribute(a.name)
        }
    }
    var old_children = Array.prototype.slice.call(old_root.childNodes);
    var new_children = Array.prototype.slice.call(new_root.childNodes);
    var old_start = 0;
    var new_start = 0;
    var old_end = old_children.length;
    var new_end = new_children.length;
    while (old_start < old_end || new_start < new_end) {
        if (old_start === old_end) {
            var target = old_children[old_end];
            while (new_start < new_end) {
                old_root.insertBefore(new_children[new_start++], target)
            }
            break
        }
        if (new_start === new_end) {
            while (old_start < old_end) {
                old_root.removeChild(old_children[old_start++])
            }
            break
        }
        if (old_children[old_start].isEqualNode(new_children[new_start])) {
            old_start++;
            new_start++;
            continue
        }
        if (old_children[old_end - 1].isEqualNode(new_children[new_end - 1])) {
            old_end--;
            new_end--;
            continue
        }
        if (old_children[old_start].isEqualNode(new_children[new_start + 1])) {
            old_root.insertBefore(new_children[new_start], old_children[old_start]);
            new_start += 2;
            old_start++;
            continue
        }
        if (old_children[old_end - 1].isEqualNode(new_children[new_end - 2])) {
            old_root.insertBefore(new_children[new_end - 1], old_children[old_end]);
            new_end -= 2;
            old_end--;
            continue
        }
        old_children[old_start] = update_in_place(old_children[old_start], new_children[new_start]);
        old_start++;
        new_start++
    }
    return old_root
};

function virtual_refresh() {
    xhr("GET", window.location.href, null, function(req) {
        var parser = new DOMParser();
        var doc = parser.parseFromString(req.response, "text/html");
        update_in_place(document.querySelector("body"), doc.body)
    })
}