https://botsailor.com/assets/js/common/load-lazy-assets.js

1window.addEventListener('load', () => {
2  const masterAssetLoader = async () => {
3    const lazyAssets = document.querySelectorAll(
4      'link[type="text/lazy-style"], script[type="text/lazy-script"]'
5    );
6
7    const loadStylesheet = (el) =>
8      new Promise((resolve, reject) => {
9        const link = document.createElement('link');
10        link.rel = 'stylesheet';
11        link.href = el.getAttribute('data-src');
12        link.onload = () => {
13          el.remove();
14          resolve();
15        };
16        link.onerror = () =>
17          reject(new Error(`Failed to load CSS: ${el.getAttribute('data-src')}`));
18        document.head.appendChild(link);
19      });
20
21    const loadExternalScript = (el) =>
22      new Promise((resolve, reject) => {
23        const script = document.createElement('script');
24        script.src = el.getAttribute('data-src');
25        script.async = false;
26        script.onload = () => {
27          el.remove();
28          resolve();
29        };
30        script.onerror = () =>
31          reject(new Error(`Failed to load JS: ${el.getAttribute('data-src')}`));
32        document.body.appendChild(script);
33      });
34
35    const runInlineScript = (el) => {
36      const text = el.textContent;
37      if (text && text.trim()) {
38        const inline = document.createElement('script');
39        inline.text = text;
40        document.body.appendChild(inline);
41      }
42      el.remove();
43    };
44
45    for (const el of lazyAssets) {
46      try {
47        if (el.matches('link[type="text/lazy-style"]')) {
48          await loadStylesheet(el);
49          continue;
50        }
51
52        const src = el.getAttribute('data-src');
53        if (!src) {
54          runInlineScript(el);
55        } else {
56          await loadExternalScript(el);
57        }
58      } catch (e) {
59        console.error(e);
60      }
61    }
62
63    window.dispatchEvent(new CustomEvent('lazyassets:ready'));
64  };
65
66  if ('requestIdleCallback' in window) {
67    requestIdleCallback(masterAssetLoader, { timeout: 2000 });
68  } else {
69    setTimeout(masterAssetLoader, 50);
70  }
71});