Function preload

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js:4695–4771

(href: string, as: string, options?: ?PreloadImplOptions)

}

function preload(href: string, as: string, options?: ?PreloadImplOptions) {
  previousDispatcher.L(/* preload */ href, as, options);
  const ownerDocument = getGlobalDocument();
  if (ownerDocument && href && as) {
    let preloadSelector = `link[rel="preload"][as="${escapeSelectorAttributeValueInsideDoubleQuotes(
      as,
    )}"]`;
    if (as === 'image') {
      if (options && options.imageSrcSet) {
        preloadSelector += `[imagesrcset="${escapeSelectorAttributeValueInsideDoubleQuotes(
          options.imageSrcSet,
        )}"]`;
        if (typeof options.imageSizes === 'string') {
          preloadSelector += `[imagesizes="${escapeSelectorAttributeValueInsideDoubleQuotes(
            options.imageSizes,
          )}"]`;
        }
      } else {
        preloadSelector += `[href="${escapeSelectorAttributeValueInsideDoubleQuotes(
          href,
        )}"]`;
      }
    } else {
      preloadSelector += `[href="${escapeSelectorAttributeValueInsideDoubleQuotes(
        href,
      )}"]`;
    }
    // Some preloads are keyed under their selector. This happens when the preload is for
    // an arbitrary type. Other preloads are keyed under the resource key they represent a preload for.
    // Here we figure out which key to use to determine if we have a preload already.
    let key = preloadSelector;
    switch (as) {
      case 'style':
        key = getStyleKey(href);
        break;
      case 'script':
        key = getScriptKey(href);
        break;
    }
    if (!preloadPropsMap.has(key)) {
      const preloadProps = Object.assign(
        ({
          rel: 'preload',
          // There is a bug in Safari where imageSrcSet is not respected on preload links
          // so we omit the href here if we have imageSrcSet b/c safari will load the wrong image.
          // This harms older browers that do not support imageSrcSet by making their preloads not work
          // but this population is shrinking fast and is already small so we accept this tradeoff.
          href:
            as === 'image' && options && options.imageSrcSet ? undefined : href,
          as,
        }: PreloadProps),
        options,
      );
      preloadPropsMap.set(key, preloadProps);

      if (null === ownerDocument.querySelector(preloadSelector)) {
        if (
          as === 'style' &&