Function appendMissingHeadersDetection

packages/common/http/src/transfer_cache.ts:477–518

* This function will add a proxy to an HttpHeader to intercept calls to get/has * and log a warning if the header entry requested has been removed

(
  url: string,
  headers: HttpHeaders,
  headersToInclude: string[],
)

 * and log a warning if the header entry requested has been removed
 */
function appendMissingHeadersDetection(
  url: string,
  headers: HttpHeaders,
  headersToInclude: string[],
): HttpHeaders {
  const warningProduced = new Set();
  return new Proxy<HttpHeaders>(headers, {
    get(target: HttpHeaders, prop: keyof HttpHeaders): unknown {
      const value = Reflect.get(target, prop);
      const methods: Set<keyof HttpHeaders> = new Set(['get', 'has', 'getAll']);

      if (typeof value !== 'function' || !methods.has(prop)) {
        return value;
      }

      return (headerName: string) => {
        // We log when the key has been removed and a warning hasn't been produced for the header
        const key = (prop + ':' + headerName).toLowerCase(); // e.g. `get:cache-control`
        if (!headersToInclude.includes(headerName) && !warningProduced.has(key)) {
          warningProduced.add(key);
          const truncatedUrl = truncateMiddle(url);

          console.warn(
            formatRuntimeError(
              RuntimeErrorCode.HEADERS_ALTERED_BY_TRANSFER_CACHE,
              `Angular detected that the \`${headerName}\` header is accessed, but the value of the header ` +
                `was not transferred from the server to the client by the HttpTransferCache. ` +
                `To include the value of the \`${headerName}\` header for the \`${truncatedUrl}\` request, ` +
                `use the \`includeHeaders\` list. The \`includeHeaders\` can be defined either ` +
                `on a request level by adding the \`transferCache\` parameter, or on an application ` +
                `level by adding the \`httpCacheTransfer.includeHeaders\` argument to the ` +
                `\`provideClientHydration()\` call. `,
            ),
          );
        }

        // invoking the original method
        return (value as Function).apply(target, [headerName]);
      };
    },
  });
}

function mapRequestOriginUrl(url: string, originMap: Record<string, string>): string {
  const origin = new URL(url, 'resolve://').origin;