Quick and dirty Node.js library for LTA's Arrival API

arrival.js

"use strict";
/* jshint esversion: 6 */
/* jshint node: true */

const fs = require("fs");
const url = require("url");
const http = require("http");
const path = require("path");
const querystring = require("querystring");

const ENDPOINT = "http://datamall2.mytransport.sg/ltaodataservice/";
const SERVICE = "BusArrival";
const AUTH = JSON.parse(fs.readFileSync(path.join(__dirname, "auth.json"), "utf8"));

const BASE_URL = url.parse(url.resolve(ENDPOINT, SERVICE));
const STOP_URL = (stopID) => Object.assign(BASE_URL, {
	path: BASE_URL.path + "?" + querystring.stringify({ BusStopID: stopID, SST: true })
});
const STOP_OPTIONS = (stopID) => Object.assign(STOP_URL(stopID), {
	headers: Object.assign({ "Accept": "application/json" }, AUTH)
});

const UPDATE_DELAY = 30 * 1000;
const UPDATE_DELAY_ERROR = 5 * 1000;

class ArrivalAPI {

	constructor(onDemand) {
		this.onDemand = onDemand;
		this._cached = {};
		this._timers = {};
	}

	get(stopID, cb) {
		if (this.onDemand) {
			this._fetch(stopID, cb);
		}
		else {
			this._fetchCached(stopID, cb);
		}
	}

	_fetchCached(stopID, cb) {
		stopID = parseInt(stopID);
		if (this._cached[stopID]) {
			cb(this._cached[stopID][0], this._cached[stopID][1]);
		}
		else {
			this._fetch(stopID, (data, err) => {
				this._cached[stopID] = [data, err];
				this._fetchCached(stopID, cb);
				this._keepUpdated(stopID, UPDATE_DELAY);
			});
		}
	}

	_keepUpdated(stopID, delay) {
		//stopID = parseInt(stopID); // Any internal call should not be a string
		this._timers[stopID] = setTimeout(() => {
			this._fetch(stopID, (data, err) => {
				this._cached[stopID] = [data, err];
				if (err) {
					return this._keepUpdated(stopID, UPDATE_DELAY_ERROR);
				}
				return this._keepUpdated(stopID, UPDATE_DELAY);
			});
		}, delay);
	}

	_fetch(stopID, cb) {
		http.get(STOP_OPTIONS(stopID), (res) => {
			var buff = "";
			res.on("data", (chunk) => {
				buff += chunk;
			});
			res.on("error", (err) => {
				cb({}, err);
				console.error(err);
			});
			res.on("end", () => {
				try {
					if (res.statusCode == 200) {
						cb(JSON.parse(buff));
					}
					else {
						cb({}, {
							status: res.statusCode
						});
						console.error(buff, res.statusCode);
					}
				}
				catch (err) {
					cb({}, err);
					console.error(err);
				}
			});
		});
	}

}

module.exports = ArrivalAPI;