Function appendPreloadHintsToHtml

packages/angular/ssr/src/app.ts:536–550

* Appends module preload hints to an HTML string for specified JavaScript resources. * This function enhances the HTML by injecting ` ` elements * for each provided resource, allowing browsers to preload the specified JavaScript * modules for better performance. * * @pa

(html: string, preload: readonly string[])

 *          If `</body>` is not found, the links are not added.
 */
function appendPreloadHintsToHtml(html: string, preload: readonly string[]): string {
  const bodyCloseIdx = html.lastIndexOf('</body>');
  if (bodyCloseIdx === -1) {
    return html;
  }

  // Note: Module preloads should be placed at the end before the closing body tag to avoid a performance penalty.
  // Placing them earlier can cause the browser to prioritize downloading these modules
  // over other critical page resources like images, CSS, and fonts.
  return [
    html.slice(0, bodyCloseIdx),
    ...preload.map((val) => `<link rel="modulepreload" href="${val}">`),
    html.slice(bodyCloseIdx),
  ].join('\n');
}