fetch to gm

fetch.js

function fetch (url, { method = "GET", headers = {}, body = null } = {}) {
  return new Promise((resolve, reject) => {
    if ("GET" === method) {
      if (body) {
        url += "?" + body;
        body = null;
      }
      delete headers["Content-Type"];
    }
    console.log(method, url, body, headers);

    GM.xmlHttpRequest({
      url,
      method,
      headers,
      data: body,
      onload: xhr => {
        const { status, statusText, responseHeaders } = xhr;
        const headers = new Headers();
        responseHeaders.split("\r\n").forEach(v => {
          if (/([^:]+):\s*(.+)/.test(v)) {
            headers.append(RegExp.$1, RegExp.$2);
          }
        });

        let body = xhr.response;
        if (204 === status) {
          body = null;
        }
        console.log("RESPONSE", status, statusText);
        resolve(new window.Response(body, {
          status,
          statusText,
          headers
        }));
      },
      onerror: xhr => {
        reject("ERROR");
      },
      ontimeout: xhr => {
        reject("TIMEOUT");
      },
      onabort: xhr => {
        reject("ABORT");
      }
    });
  });
};