monkeypatch.ts

function monkeyPatchXMLHttpRequest() {
  const originalSetRequestHeader = XMLHttpRequest.prototype.setRequestHeader;
  const originalAddEventListener = XMLHttpRequest.prototype.addEventListener;

  XMLHttpRequest.prototype.setRequestHeader = function (header: string, value: string): void {
    if (header.toLowerCase() === "x-passphrase") {
      this.passphrase = value;
    } else {
      originalSetRequestHeader.apply(this, [ header, value ]);
    }
  };

  XMLHttpRequest.prototype.addEventListener = function (type, listener, options?) {
    if (type === "load") {
      originalAddEventListener.apply(this, [ "load", async function (event: ProgressEvent) {
        if (this.passphrase) {
          Object.defineProperty(this, "response", { writable: true });
          Object.defineProperty(this, "responseText", { writable: true });

          // this.response is `undefined` here.  Have I got mixed up with my `this`s?

          listener(event);
        } else {
          listener(event);
        }
      } ]);
    } else {
      originalAddEventListener.apply(this, [ type, listener, options ]);
    }
  };
}