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/**
* A lightweight youtube embed. Still should feel the same to the user, just MUCH faster to initialize and paint.
*
* Thx to these as the inspiration
* https://storage.googleapis.com/amp-vs-non-amp/youtube-lazy.html
* https://autoplay-youtube-player.glitch.me/
*
* Once built it, I also found these:
* https://github.com/ampproject/amphtml/blob/master/extensions/amp-youtube (👍👍)
* https://github.com/Daugilas/lazyYT
* https://github.com/vb/lazyframe
*/
class LiteYTEmbed extends HTMLElement {
connectedCallback() {
this.videoId = this.getAttribute('videoid');
let playBtnEl = this.querySelector('.lty-playbtn');
// A label for the button takes priority over a [playlabel] attribute on the custom-element
this.playLabel = (playBtnEl && playBtnEl.textContent.trim()) || this.getAttribute('playlabel') || 'Play';
this.dataset.title = this.getAttribute('title') || "";
/**
* Lo, the youtube poster image! (aka the thumbnail, image placeholder, etc)
*
* See https://github.com/paulirish/lite-youtube-embed/blob/master/youtube-thumbnail-urls.md
*/
if (!this.style.backgroundImage) {
this.style.backgroundImage = `url("https://i.ytimg.com/vi/${this.videoId}/hqdefault.jpg")`;
this.upgradePosterImage();
}
// Set up play button, and its visually hidden label
if (!playBtnEl) {
playBtnEl = document.createElement('button');
playBtnEl.type = 'button';
playBtnEl.classList.add('lty-playbtn');
this.append(playBtnEl);
}
if (!playBtnEl.textContent) {
const playBtnLabelEl = document.createElement('span');
playBtnLabelEl.className = 'lyt-visually-hidden';
playBtnLabelEl.textContent = this.playLabel;
playBtnEl.append(playBtnLabelEl);
}
this.addNoscriptIframe();
// for the PE pattern, change anchor's semantics to button
if(playBtnEl.nodeName === 'A'){
playBtnEl.removeAttribute('href');
playBtnEl.setAttribute('tabindex', '0');
playBtnEl.setAttribute('role', 'button');
// fake button needs keyboard help
playBtnEl.addEventListener('keydown', e => {
if( e.key === 'Enter' || e.key === ' ' ){
e.preventDefault();
this.activate();
}
});
}
// On hover (or tap), warm up the TCP connections we're (likely) about to use.
this.addEventListener('pointerover', LiteYTEmbed.warmConnections, {once: true});
this.addEventListener('focusin', LiteYTEmbed.warmConnections, {once: true});
// Once the user clicks, add the real iframe and drop our play button
// TODO: In the future we could be like amp-youtube and silently swap in the iframe during idle time
// We'd want to only do this for in-viewport or near-viewport ones: https://github.com/ampproject/amphtml/pull/5003
this.addEventListener('click', this.activate);
// Chrome & Edge desktop have no problem with the basic YouTube Embed with ?autoplay=1
// However Safari desktop and most/all mobile browsers do not successfully track the user gesture of clicking through the creation/loading of the iframe,
// so they don't autoplay automatically. Instead we must load an additional 2 sequential JS files (1KB + 165KB) (un-br) for the YT Player API
// TODO: Try loading the the YT API in parallel with our iframe and then attaching/playing it. #82
this.needsYTApi = this.hasAttribute("js-api") || navigator.vendor.includes('Apple') || navigator.userAgent.includes('Mobi');
}
/**
* Add a <link rel={preload | preconnect} ...> to the head
*/
static addPrefetch(kind, url, as) {
const linkEl = document.createElement('link');
linkEl.rel = kind;
linkEl.href = url;
if (as) {
linkEl.as = as;
}
document.head.append(linkEl);
}
/**
* Begin pre-connecting to warm up the iframe load
* Since the embed's network requests load within its iframe,
* preload/prefetch'ing them outside the iframe will only cause double-downloads.
* So, the best we can do is warm up a few connections to origins that are in the critical path.
*
* Maybe `<link rel=preload as=document>` would work, but it's unsupported: http://crbug.com/593267
* But TBH, I don't think it'll happen soon with Site Isolation and split caches adding serious complexity.
*/
static warmConnections() {
if (LiteYTEmbed.preconnected) return;
// The iframe document and most of its subresources come right off youtube.com
LiteYTEmbed.addPrefetch('preconnect', 'https://www.youtube-nocookie.com');
// The botguard script is fetched off from google.com
LiteYTEmbed.addPrefetch('preconnect', 'https://www.google.com');
// Not certain if these ad related domains are in the critical path. Could verify with domain-specific throttling.
LiteYTEmbed.addPrefetch('preconnect', 'https://googleads.g.doubleclick.net');
LiteYTEmbed.addPrefetch('preconnect', 'https://static.doubleclick.net');
LiteYTEmbed.preconnected = true;
}
fetchYTPlayerApi() {
if (window.YT || (window.YT && window.YT.Player)) return;
this.ytApiPromise = new Promise((res, rej) => {
var el = document.createElement('script');
el.src = 'https://www.youtube.com/iframe_api';
el.async = true;
el.onload = _ => {
YT.ready(res);
};
el.onerror = rej;
this.append(el);
});
}
/** Return the YT Player API instance. (Public L-YT-E API) */
async getYTPlayer() {
if(!this.playerPromise) {
await this.activate();
}
return this.playerPromise;
}
async addYTPlayerIframe() {
this.fetchYTPlayerApi();
await this.ytApiPromise;
const videoPlaceholderEl = document.createElement('div')
this.append(videoPlaceholderEl);
const paramsObj = Object.fromEntries(this.getParams().entries());
this.playerPromise = new Promise(resolve => {
let player = new YT.Player(videoPlaceholderEl, {
width: '100%',
videoId: this.videoId,
playerVars: paramsObj,
events: {
'onReady': event => {
event.target.playVideo();
resolve(player);
}
}
});
});
}
// Add the iframe within <noscript> for indexability discoverability. See https://github.com/paulirish/lite-youtube-embed/issues/105
addNoscriptIframe() {
const iframeEl = this.createBasicIframe();
const noscriptEl = document.createElement('noscript');
// Appending into noscript isn't equivalant for mysterious reasons: https://html.spec.whatwg.org/multipage/scripting.html#the-noscript-element
noscriptEl.innerHTML = iframeEl.outerHTML;
this.append(noscriptEl);
}
getParams() {
const params = new URLSearchParams(this.getAttribute('params') || []);
params.append('autoplay', '1');
params.append('playsinline', '1');
return params;
}
async activate(){
if (this.classList.contains('lyt-activated')) return;
this.classList.add('lyt-activated');
if (this.needsYTApi) {
return this.addYTPlayerIframe(this.getParams());
}
const iframeEl = this.createBasicIframe();
this.append(iframeEl);
// Set focus for a11y
iframeEl.focus();
}
createBasicIframe(){
const iframeEl = document.createElement('iframe');
iframeEl.width = 560;
iframeEl.height = 315;
// No encoding necessary as [title] is safe. https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html#:~:text=Safe%20HTML%20Attributes%20include
iframeEl.title = this.playLabel;
iframeEl.allow = 'accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture';
iframeEl.allowFullscreen = true;
// AFAIK, the encoding here isn't necessary for XSS, but we'll do it only because this is a URL
// https://stackoverflow.com/q/64959723/89484
iframeEl.src = `https://www.youtube-nocookie.com/embed/${encodeURIComponent(this.videoId)}?${this.getParams().toString()}`;
return iframeEl;
}
/**
* In the spirit of the `lowsrc` attribute and progressive JPEGs, we'll upgrade the reliable
* poster image to a higher resolution one, if it's available.
* Interestingly this sddefault webp is often smaller in filesize, but we will still attempt it second
* because getting _an_ image in front of the user if our first priority.
*
* See https://github.com/paulirish/lite-youtube-embed/blob/master/youtube-thumbnail-urls.md for more details
*/
upgradePosterImage() {
// Defer to reduce network contention.
setTimeout(() => {
const webpUrl = `https://i.ytimg.com/vi_webp/${this.videoId}/sddefault.webp`;
const img = new Image();
img.fetchPriority = 'low'; // low priority to reduce network contention
img.referrerpolicy = 'origin'; // Not 100% sure it's needed, but https://github.com/ampproject/amphtml/pull/3940
img.src = webpUrl;
img.onload = e => {
// A pretty ugly hack since onerror won't fire on YouTube image 404. This is (probably) due to
// Youtube's style of returning data even with a 404 status. That data is a 120x90 placeholder image.
// … per "annoying yt 404 behavior" in the .md
const noAvailablePoster = e.target.naturalHeight == 90 && e.target.naturalWidth == 120;
if (noAvailablePoster) return;
this.style.backgroundImage = `url("${webpUrl}")`;
}
}, 100);
}
}
// Register custom element
customElements.define('lite-youtube', LiteYTEmbed);
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