Slack App in Google Apps Script

SlackApp.gs

// *** Request Verification ***
// The currently recommended way is to verify request signature: https://api.slack.com/authentication/verifying-requests-from-slack
// Unfortunately, GAS web apps don"t have means to access request headers. Thus, this example uses the old way to verify requests.
// >>> Settings > Basic Information > App Credentials > Verification Token
const legacyVerificationToken = PropertiesService.getScriptProperties().getProperty("SLACK_VERIFICATION_TOKEN");

// *** OAuth Access Token ***
// Install your Slack app into its development workspace.
// >>> Settings > Install App > Bot User OAuth Access Token
const token = PropertiesService.getScriptProperties().getProperty("SLACK_BOT_TOKEN");

// Example modal view
// Build your own modals here: https://app.slack.com/block-kit-builder
const modalView = {
  "type": "modal",
  "callback_id": "modal-id",
  "submit": {
    "type": "plain_text",
    "text": "Submit"
  },
  "close": {
    "type": "plain_text",
    "text": "Cancel"
  },
  "title": {
    "type": "plain_text",
    "text": "Notification settings"
  },
  "blocks": [
    {
      "type": "actions",
      "block_id": "b",
      "elements": [
        {
          "type": "checkboxes",
          "action_id": "a",
          "options": [
            {
              "text": {
                "type": "plain_text",
                "text": "New tasks"
              },
              "value": "tasks",
              "description": {
                "type": "plain_text",
                "text": "When new tasks are added to project"
              }
            },
            {
              "text": {
                "type": "plain_text",
                "text": "New comments"
              },
              "value": "comments",
              "description": {
                "type": "plain_text",
                "text": "When new comments are added"
              }
            },
            {
              "text": {
                "type": "plain_text",
                "text": "Project updates"
              },
              "value": "updates",
              "description": {
                "type": "plain_text",
                "text": "When project is updated"
              }
            }
          ]
        }
      ]
    }
  ]
};

// POST request handler
function doPost(e) {
  console.log(`Incoming form request data: ${JSON.stringify(e.postData)}`);

  if (typeof e.postData === "undefined") {
    return ack("invalid request");
  }

  if (e.postData.type === "application/json") {
    // ----------------------------
    // Events API / url_verification
    // ----------------------------

    const payload = JSON.parse(e.postData.getDataAsString());
    if (payload.token !== legacyVerificationToken) {
      console.log(`Invalid verification token detected (actual: ${payload.token}, expected: ${legacyVerificationToken})`);
      return ack("invalid request");
    }
    if (typeof payload.challenge !== "undefined") {
      return ack(payload.challenge);
    }
    console.log(`Events API payload: ${JSON.stringify(payload)}`);

    // -------------------------------------------------------------
    // TODO: Implement your own logic here
    if (typeof payload.event.channel !== "undefined") {
      callWebApi(token, "chat.postMessage", {
        channel: payload.event.channel,
        text: "Hi there!"
      });
    }
    // -------------------------------------------------------------

    return ack("");

  } else if (e.postData.type === "application/x-www-form-urlencoded") {
    if (typeof e.parameters.payload !== "undefined") {
      // ----------------------------
      // Interactivity & Shortcuts
      // ----------------------------

      const payload = JSON.parse(e.parameters.payload[0]);
      if (payload.token !== legacyVerificationToken) {
        console.log(`Invalid verification token detected (actual: ${payload.token}, expected: ${legacyVerificationToken})`);
        return ack("invalid request");
      }
      console.log(`Interactivity payload: ${JSON.stringify(payload)}`);

      // -------------------------------------------------------------
      // TODO: Implement your own logic here
      if (payload.type === "shortcut") {
        if (payload.callback_id === "gas") {
          callWebApi(token, "views.open", {
            trigger_id: payload.trigger_id,
            user_id: payload.user.id,
            view: JSON.stringify(modalView)
          });
        }
      } else if (payload.type === "message_action") {
        if (payload.callback_id === "gas-msg") {
          respond(payload.response_url, "Thanks for running a message shortcut!");
        }
      } else if (payload.type === "block_actions") {
        console.log(`Action data: ${JSON.stringify(payload.actions[0])}`);
      } else if (payload.type === "view_submission") {
        if (payload.view.callback_id === "modal-id") {
          const stateValues = payload.view.state.values;
          console.log(`View submssion data: ${JSON.stringify(stateValues)}`);
          return ack("");
        }
      }
      // -------------------------------------------------------------

    } else if (typeof e.parameters.command !== "undefined") {
      // ----------------------------
      // Slash Commands
      // ----------------------------

      const payload = {}
      for (const [key, value] of Object.entries(e.parameters)) {
        payload[key] = value[0];
      }
      if (payload.token !== legacyVerificationToken) {
        console.log(`Invalid verification token detected (actual: ${payload.token}, expected: ${legacyVerificationToken})`);
        return ack("invalid request");
      }
      console.log(`Slash command payload: ${JSON.stringify(payload)}`);

      // -------------------------------------------------------------
      // TODO: Implement your own logic here
      if (payload.command === "/gas") {
        return ack("Hi there!");
      }
      // -------------------------------------------------------------
    }

  }
  // acknowledge the request
  return ack("");
}

function callWebApi(token, apiMethod, payload) {
  const response = UrlFetchApp.fetch(
    `https://www.slack.com/api/${apiMethod}`,
    {
      method: "post",
      contentType: "application/x-www-form-urlencoded",
      headers: { "Authorization": `Bearer ${token}` },
      payload: payload,
    }
  );
  console.log(`Web API (${apiMethod}) response: ${response}`)
  return response;
}

function respond(responseUrl, payload) {
  var responseBody = payload;
  if (typeof payload === "string") {
    responseBody = { "text": payload };
  }
  const response = UrlFetchApp.fetch(responseUrl, {
    method: "post",
    contentType: "application/json; charset=utf-8",
    payload: JSON.stringify(responseBody),
  }
  );
  console.log(response);
}

function ack(payload) {
  if (typeof payload === "string") {
    return ContentService.createTextOutput(payload);
  } else {
    return ContentService.createTextOutput(JSON.stringify(payload));
  }
}

@seratch thank you for this great resource.
It would be great if you could make one update that I identified in my project and is required for the use of the response_action method of clearing, updating, pushing modals or showing error messages:

function ack(payload) {
  if (typeof payload === "string") {
    return ContentService.createTextOutput(payload);
  } else {
    return ContentService.createTextOutput(JSON.stringify(payload))
               .setMimeType(ContentService.MimeType.JSON);
  }
}