In-browser Telegram bot

bot.js

import { HttpPoller } from './httpPoller.js';

function last(array) {
    return array[array.length - 1];
}

const token = "BOT_TOKEN";
const url = `https://api.telegram.org/bot${token}/getUpdates`;
const poller = new HttpPoller(url, "POST");

poller.start(data => {
    const apiResponse = JSON.parse(data.response);
    console.log(apiResponse);
    const updates = apiResponse.result;
    if (updates.length > 0) {
        data.state.body = {
            offset: last(updates).update_id + 1
        };
    }
});

httpPoller.js

export class HttpPoller {
    constructor(url, method, timeout = 30) {
        this.url = url;
        this.method = method;
        this.timeout = timeout;
        this.state = HttpPoller.STOPPED;
    }

    set timeout(value) {
        this._timeout = value * 1000;
    }

    get timeout() {
        return this._timeout;
    }

    withBody() {
        return ["POST", "PUT"].includes(this.method);
    }

    start(callback, body = null) {
        if (this.state == HttpPoller.STARTED) {
            return;
        }

        const pollerState = {
            body: body
        };
        const poller = (callback, pollerState) => {
            if (this.state == HttpPoller.STOPPED) {
                return;
            }

            const xhr = new XMLHttpRequest();
            xhr.open(this.method, this.url, true);
            xhr.timeout = this.timeout;
            xhr.onreadystatechange = (e) => {
                if (this.state == HttpPoller.STOPPED) {
                    e.target.abort();
                    return;
                }
                if(e.target.readyState == XMLHttpRequest.DONE) {
                    callback({
                        response: e.target.response,
                        state: pollerState
                    });
                    setTimeout(() => poller(callback, pollerState));
                }
            };
            xhr.onabort = () => this.stop();
            xhr.onerror = () => this.stop();
            xhr.ontimeout = () => setTimeout(() => poller(callback, pollerState));
            xhr.setRequestHeader("Content-Type", "application/json");

            if (this.withBody() && pollerState.body) {
                xhr.send(JSON.stringify(pollerState.body));
            } else {
                xhr.send();
            }
        };

        this.state = HttpPoller.STARTED;
        setTimeout(() => poller(callback, pollerState));
    }

    stop() {
        this.state = HttpPoller.STOPPED;
    }
}

HttpPoller.STOPPED = 0;
HttpPoller.STARTED = 1;