I hated all these answers because they were too short or relied on other frameworks.
Here is “one” vanilla JS way of doing this, working in Chrome, please test in other browsers:
var video = document.getElementById('video');
var source = document.createElement('source');
source.setAttribute('src', 'http://techslides.com/demos/sample-videos/small.mp4');
source.setAttribute('type', 'video/mp4');
video.appendChild(source);
video.play();
console.log({
src: source.getAttribute('src'),
type: source.getAttribute('type'),
});
setTimeout(function() {
video.pause();
source.setAttribute('src', 'http://techslides.com/demos/sample-videos/small.webm');
source.setAttribute('type', 'video/webm');
video.load();
video.play();
console.log({
src: source.getAttribute('src'),
type: source.getAttribute('type'),
});
}, 3000);
<video id="video" width="320" height="240"></video>