index.js

'use strict';

const line = require('@line/bot-sdk');
const express = require('express');
const translate = require('@vitalets/google-translate-api');

// create LINE SDK config from env variables
const config = {
  channelAccessToken: process.env.CHANNEL_ACCESS_TOKEN,
  channelSecret: process.env.CHANNEL_SECRET,
};

// create LINE SDK client
const client = new line.Client(config);

// create Express app
// about Express itself: https://expressjs.com/
const app = express();

// register a webhook handler with middleware
// about the middleware, please refer to doc
app.post('/webhook', line.middleware(config), (req, res) => {
  Promise
    .all(req.body.events.map(handleEvent))
    .then((result) => res.json(result));
});

// event handler
function handleEvent(event) {
  if (event.type !== 'message' || event.message.type !== 'text') {
    // ignore non-text-message event
    return Promise.resolve(null);
  }

  translate(event.message.text, {to: 'en'}).then(res => {
    if (res.text != event.message.text) {
      const translated = { type: 'text', text: res.text };
      return client.replyMessage(event.replyToken, translated);
    }
  }).catch(err => {
    console.error(err);
  });
}

// listen on port
const port = process.env.PORT || 3000;
app.listen(port, () => {
  console.log(`listening on ${port}`);
});