src/lib/perf/font-preloads.ts

Linked repository file from this build.

/** React/Next can move resource hints from HTML into HTTP Link headers. Count both. */
export function fontPreloads(html: string, linkHeader = ""): string[] {
  const urls = new Set<string>();
  for (const tag of html.match(/<link\b[^>]*>/gi) ?? []) {
    const attributes = Object.fromEntries(
      [...tag.matchAll(/([\w-]+)\s*=\s*(["'])(.*?)\2/g)].map((match) => [
        match[1].toLowerCase(),
        match[3],
      ]),
    );
    if (
      attributes.rel?.split(/\s+/).includes("preload") &&
      attributes.as === "font" &&
      attributes.href
    )
      urls.add(attributes.href);
  }
  for (const match of linkHeader.matchAll(/<([^>]+)>([^<]*)/g)) {
    const attributes = match[2].replaceAll('"', "");
    if (
      /;\s*rel=preload\s*(?:;|,|$)/i.test(attributes) &&
      /;\s*as=font\s*(?:;|,|$)/i.test(attributes)
    )
      urls.add(match[1]);
  }
  return [...urls];
}