RestAPI.gs

// Adapted from https://github.com/uu1t/gas-trello for Slack webhook

var endpoint_ = PropertiesService.getScriptProperties().getProperty('Slack_incoming_webhook');

/**
 * Send GET request.
 * @param {string} path
 * @param {{ [key: string]: string }} [params]
 * @return {any}
 */
function get(path, params) {
  return rest('GET', path, params);
}

/**
 * Send POST request.
 * @param {string} path
 * @param {{ [key: string]: string }} [params]
 * @return {any}
 */
function post(path, params, body = '') {
  return rest('POST', path, params, body);
}

/**
 * Send PUT request.
 * @param {string} path
 * @param {{ [key: string]: string }} [params]
 * @param {string} body of the request
 * @return {any}
 */
function put(path, params, body = '') {
  return rest('PUT', path, params, body);
}

/**
 * Send DELETE request.
 * @param {string} path
 * @param {{ [key: string]: string }} [params]
 * @return {any}
 */
function del(path, params) {
  return rest('DELETE', path, params);
}

/**
 * Send REST request.
 * @param {'GET' | 'POST' | 'PUT' | 'DELETE'} method HTTP method to use when making the request to the Trello API
 * @param {string} path API path to use, such as "members/me"
 * @param {{ [key: string]: string }} [params={}] Query parameters to the API path, such as { fields: "username,fullName" }
 * @param {string} request body
 * @return {any} Result object
 */
function rest(method, path, params, bodydata = '') {
  params = params || {};

  path = ""; //Slack webhooks have a fixed endpoint (there is no API path)

  // Get rid of any leading /
  path = path.replace(/^\/*/, '');

  var url = endpoint_ + path + toQuery_(params); 

  var response = UrlFetchApp.fetch(url, {
    method: method,
    contentType: 'application/json',
    payload: bodydata
  });

  response = response.getContentText();
  try{
    //if the response is json, try to parse it
    response = JSON.parse();
  }
  catch(e){}
  return response;
}

/**
 * @param {{ [key: string]: string }} params
 * @return {string}
 */
function toQuery_(params) {
  return (
    '?' +
    Object.keys(params)
      .map(function(key) {
        return key + '=' + encodeURIComponent(params[key]);
      })
      .join('&')
  );
}