index.js

const express = require("express");
const bodyParser = require("body-parser");
const crypto = require('crypto');
const request = require("request-promise");
const LINE_MESSAGING_API = 'https://api.line.me/v2/bot/message';
const LINE_CHANNEL_SECRET = 'XXXXXXXXXXXXX';
const LINE_HEADER = {
    'Content-Type': 'application/json',
    'Authorization': `Bearer XXXXXXXXXXXXXXXXXXX`
};

const app = express();
app.use(bodyParser.json());

app.listen(3000, () => {
    console.log(`Server available on port 3000`);
});

app.post("/webhook", async (req, res) => {
    const body = JSON.stringify(req.body);
    const signature = crypto.createHmac('SHA256', LINE_CHANNEL_SECRET).update(body).digest('base64').toString();
    if (signature !== req.headers['x-line-signature']) {
        return res.status(401).send('Unauthorized');
    }

    let event = req.body.events[0];
    if (event.type === 'message' && event.message.type === 'sticker') {
        let keywords = event.message.keywords;
        let stickerIntent = "";
        for (let i = 0; i <= 2; i++) {
            stickerIntent += randomItem(keywords) + " ";
        }

        const newBody = {
            destination: req.body.destination,
            events: [{
                timestamp: req.body.events[0].timestamp,
                mode: req.body.events[0].mode,
                source: req.body.events[0].source,
                replyToken: req.body.events[0].replyToken,
                type: 'message',
                message: { type: 'text', id: req.body.events[0].message.id, text: stickerIntent }
            }]
        };

        req.body = newBody;
        req.headers["x-line-signature"] = crypto.createHmac('SHA256', LINE_CHANNEL_SECRET).update(JSON.stringify(newBody)).digest('base64').toString();
        req.headers["content-length"] = Buffer.byteLength(JSON.stringify(newBody), 'utf8')
        postToDialogflow(req);

    } else if (event.type === 'message' && event.message.type === 'text') {
        postToDialogflow(req);
    } else {
        reply(req);
    }

    return res.status(200).send(req.method);
});

function randomItem(items) {
    return items[Math.floor(Math.random() * items.length)];
}

const postToDialogflow = req => {
    const body = JSON.stringify(req.body);
    req.headers.host = "dialogflow.cloud.google.com";
    return request.post({
        uri: "https://dialogflow.cloud.google.com/v1/integrations/line/webhook/xxxxxx-xxxxxxxx",
        headers: req.headers,
        body: body
    });
};

const reply = req => {
    return request.post({
        uri: `${LINE_MESSAGING_API}/reply`,
        headers: LINE_HEADER,
        body: JSON.stringify({
            replyToken: req.body.events[0].replyToken,
            messages: [
                {
                    type: "text",
                    text: "Sorry, this chatbot did not support message type " + req.body.events[0].message.type
                }
            ]
        })
    });
};