This is very confusing as seen in #12974 or #12906. My suggestion would be "legacy."
}
      if (enumerableProperties.length) {
        throw new Error(
          "The `Array.prototype` contains unexpected enumerable properties: " +
            enumerableProperties.join(", ") +
            "; thus breaking e.g. `for...in` iteration of `Array`s."
        );
      }
      
      
      
      if (
        (typeof PDFJSDev === "undefined" || PDFJSDev.test("SKIP_BABEL")) &&
        typeof ReadableStream === "undefined"
      ) {
        throw new Error(
          "The browser/environment lacks native support for critical " +
            "functionality used by the PDF.js library (e.g. `ReadableStream`); " +
            "please use an `es5`-build instead."
        );
      }
    }
    var docId = docParams.docId;
    var docBaseUrl = docParams.docBaseUrl;
    var workerHandlerName = docParams.docId + "_worker";
    var handler = new MessageHandler(workerHandlerName, docId, port);
    
    
    handler.postMessageTransfers = docParams.postMessageTransfers;
    function ensureNotTerminated() {