Flex Message in LINE Messaging API

FlexMessage.js

const functions = require("firebase-functions");
const request = require("request-promise");

const LINE_MESSAGING_API = "https://api.line.me/v2/bot/message";
const LINE_HEADER = {
  "Content-Type": "application/json",
  "Authorization": "Bearer <CHANNEL-ACCESS-TOKEN>"
};

exports.AdvanceMessage = functions.https.onRequest((req, res) => {
  return request({
    method: "POST",
    uri: `${LINE_MESSAGING_API}/push`,
    headers: LINE_HEADER,
    body: JSON.stringify({
      to: "<USER-ID>",
      messages: [
        {
          type: "flex",
          altText: "Flex Message",
          contents: {
            type: "bubble",
            direction: "ltr",
            hero: {
              type: "image",
              url: "https://i1.wp.com/mobileocta.com/wp-content/uploads/2019/04/index_package_pic1.jpg",
              size: "full",
              aspectRatio: "1.51:1",
              aspectMode: "fit"
            }
          }
        }
      ]
    })
  }).then(() => {
    return res.status(200).send("Done");
  }).catch(error => {
    return Promise.reject(error);
  });
});