HTML5 video full preload in javascript

1. Download the video using fetch
2. Convert the response to a blob
3. Create an object URL from the blob (e.g. blob:http://localhost:8080/df3c4336-2d9f-4ba9-9714-2e9e6b2b8888)

async function preloadVideo(src) {
  const res = await fetch(src);
  const blob = await res.blob();
  return URL.createObjectURL(blob);
}

Usage:

const video = document.createElement("video");
video.src = await preloadVideo("https://example.com/video.mp4");

canplaythrough is the event that should fire when enough data has downloaded to play without buffering.

From the Opera teams excellent (although maybe very slightly dated now) resource Everything you need to know about HTML5 video and audio

If the load is successful, whether using the src attribute or using source elements, then as data is being downloaded, progress events are fired. When enough data has been loaded to determine the video's dimensions and duration, a loadedmetadata event is fired. When enough data has been loaded to render a frame, the loadeddata event is fired. When enugh data has been loaded to be able to play a little bit of the video, a canplay event is fired. When the browser determines that it can play through the whole video without stopping for downloading more data, a canplaythrough event is fired; this is also when the video starts playing if it has a autoplay attribute.

'canplaythrough' support matrix available here: https://caniuse.com/mdn-api_htmlmediaelement_canplaythrough_event

You can get around the support limitations by binding the load element to the same function, as it will trigger on those.

This will load the entire video in JavaScript

var r = new XMLHttpRequest();
r.onload = function() {
    myVid.src = URL.createObjectURL(r.response);
    myVid.play();
};
if (myVid.canPlayType('video/mp4;codecs="avc1.42E01E, mp4a.40.2"')) {
    r.open("GET", "slide.mp4");
}
else {
    r.open("GET", "slide.webm");
}

r.responseType = "blob";
r.send();