JSFiddle - React, Tailwind, and code Playground

HTML

<video id="viddy" autoplay>
    <source src="http://media.w3.org/2010/05/sintel/trailer.webm" type='video/webm' />
    <source src="http://www.w3schools.com/html/movie.mp4" type="video/mp4" />
</video>

JavaScript

function init() {
    var video = document.getElementById('viddy');
    video.addEventListener('loadedmetadata', function(e){
        var dimensions = [video.videoWidth, video.videoHeight];
        alert(dimensions);
    });
}

document.addEventListener("DOMContentLoaded", init, false);