_fetchJSON.js

  _fetchJSON = async (url, json=null, headers={}) => {
    if (headers["Content-Type"] === undefined) {
      headers["Content-Type"] = "application/json";
    }
    else if (headers["Content-Type"] !== "application/json") {
      throw new Error(`Not JSON content-type: ${headers["Content-Type"]}`);
    }
    const requestHeaders = new Headers(headers);

    const requestOptions = { mode: "no-cors", headers: requestHeaders }
    if (json == null) {
      requestOptions.method = "GET";
    }
    else {
      requestOptions.method = "POST";
      requestOptions.body = JSON.stringify(json);
    }

    console.log("****" + requestOptions.headers.get("content-type"));
    // emits: application/json

    const request = new Request(url, requestOptions);

    console.log(">>>>" + request.headers.get("content-type"));
    // emits: text/plain;charset=UTF-8

    const response = await this._fetch(request);

    const responseContentType = response.headers.get("Content-Type");
    if (responseContentType !== "application/json") {
      throw new Error(`Response type is not JSON: ${responseContentType}`);
    }

    return await response.json();
  }