Writing a Telegram bot with SvelteKit and Vercel

Let's write a Telegram bot with SvelteKit. The bot will run in Vercel, which provides zero-configuration support for SvelteKit.

Assume we already have a Telegram bot registered. Its token is in the BOT_TOKEN environment variable.

Our bot will use a webhook type of deployment, not polling.

The heart of the Telegram bot operating with the webhook is the POST endpoint, receiving callbacks from the Telegram servers. After the POST endpoint receives the request and deserializes the payload from JSON, it passes it to the "node-telegram-bot-api" library to handle.

Our bot will be trivial only to demonstrate how to handle the requests from Telegram in SvelteKit. It will echo back the received text messages.

After creating a project with "npm create svelte", install the Telegram library with "node-telegram-bot-api" and add the following configuration to "svelte.config.js":

const config = {
        ...
        kit: {
            ...
            csrf: {
                checkOrigin: false,
            }
       }
};

let's create a "+server.ts" file in the "./routes" directory.

import { bot } from "./bot";

export async function POST({ request }) {
    try {
        bot.processUpdate(await request.json());
        return new Response("", { status: 200 });
    } catch (e) {
        console.error(e);
        return new Response("", { status: 500 });
    }
}

The "bot.ts" file implements the echoing of incoming messages.

export const bot = new TelegramBot(BOT_TOKEN);
console.log("bot", BOT_TOKEN);

export const history: any[] = [];
bot.on("message", async (msg) => {
    try {
        await bot.sendMessage(msg.chat.id, msg.text + ", and?");
        history.push({ when: new Date(), msg });
    } catch (e) {
        console.error(e);
    }
});

That's it.

Once the code is pushed to a git repo and deployed by Vercel, for example, on "https://sveltekit-bot.vercel.app", we need to set a webhook allowing Telegram to deliver the updates to the bot.

It needs to be done just once:

curl -X POST https://api.telegram.org/bot$(BOT_TOKEN)/setWebhook \
    -H "Content-type: application/json" \
    -d '{"url": "https://sveltekit-bot.vercel.app"}'

After that, we can start chatting with the bot!

NOTE: It is important to understand that this bot deployment to Vercel is stateless. When there is no traffic, Vercel stops the endpoint and kicks it off again when the traffic is coming. There is no data persistence
in the bot with this implementation. In a real bot, we may use external storage, like a database, to persist the bot's state.

The full source of the project is available at
https://github.com/begoon/sveltekit-bot/.

In the project, there is a trivial in-memory storage for messages and a page to print them for demonstration purposes. After the inactivity period, the deployment restarts by Vercel, and the message storage resets.