JSFiddle - React, Tailwind, and code Playground

HTML

<button id="start_button">Start</button>

	<div id='progressbar-wrapper' style="position: fixed;top: 80px;display: block;margin: 0 auto;width: 100%;height: 30px;opacity: 1;zIndex: 9998">
		<div id='progressbar-outer' style="display: table;margin: 0 auto;background-color: #FFFFFF;border: 5px solid #000000;width: 50%;height: 30px;opacity: 1;zIndex: 9998">
			<div id='progressbar' style="float:left: 50;width: 0;height: 30px;background-color:#000000;border: 0;opacity: 1;zIndex: 99999">
			</div>
		</div>
	</div>

	<div id="loading-animation" style="position: fixed;top: 150px;left: 0;height: 120px;width: 100%;font-size: 100px;line-height: 120px;text-align: center;color: #000000;zIndex: 9999;">
	    ...loading...<br /><small>Please be patient.</small>
	</div>

JavaScript

var progressbar = {};

$(function () {

	progressbar = {

        /** initial progress */
        progress: 0,

        /** maximum width of progressbar */
        progress_max: 0,

        /** The inner element of the progressbar (filled box). */
        $progress_bar: $('#progressbar'),

        /** Method to set the progressbar.
         */
        set: function (num) {
            if (this.progress_max && num) {
                this.progress = num / this.progress_max * 100;
                console.log('percent: ' + this.progress + '% - ' + num + '/' + this.progress_max);
                this.$progress_bar.width(String(this.progress) + '%');
            }
        }

	};

});

function loop(i, iterations) {
    while (iterations % ++i !== 100 && i <= iterations);
    
    if(i <= iterations) {
        requestAnimationFrame(function() {
            progressbar.set(i);
            loop(i, iterations);
        });
    }
}


$('#start_button').on('click', function () {

	var iterations = 1000000000;

	progressbar.progress_max = iterations;

    loop(0, iterations);

});