Show reading progress on scroll

prgrss.js

(function() {
    var willHide = false;

    var msgBox = document.createElement('div');
    msgBox.style.position = 'fixed';
    msgBox.style.top = '10px';
    msgBox.style.right = '10px';
    msgBox.style.display = 'none';
    document.body.appendChild(msgBox);

    window.addEventListener('scroll', function() {
        var percentLeft = (window.scrollY / (document.body.scrollHeight - window.innerHeight) * 100).toPrecision(3) + '%';

        if (msgBox.style.display === 'none') {
            msgBox.style.display = 'block';
        }

        msgBox.innerText = percentLeft;

        if (!willHide) {
            willHide = true;
            setTimeout(function() {
                msgBox.style.display = 'none';
                willHide = false;
            }, 1000);
        }
    });
})();