zaKolikDnes

Tracks the maximum fuel prices (Natural 95 gasoline, diesel) published by the Czech Ministry of Finance in the Cenový věstník, and shows the current values plus a 30-day history chart.

Next.js (App Router) app deployed on Vercel with Neon Postgres for storage and Vercel Cron for scheduled scraping.

Stack

• Next.js 14 + TypeScript + Tailwind
• Neon Postgres (@neondatabase/serverless)
• axios, pdf-parse, cheerio
• Recharts (history chart)
• pino (logging)
• Vitest (unit tests)
• Vercel Analytics

Local development

npm install
npm run dev

You need a DATABASE_URL pointing to a Neon Postgres database (set in .env or via vercel env pull).

Open http://localhost:3000. Initially the page shows "no data" — trigger a scrape:

curl -X POST -H "x-admin-token: change-me" http://localhost:3000/api/refresh

Database migration

Run once to create the prices table:

npm run db:migrate

API

Method	Path	Description
GET	/api/latest	Most recent stored prices (404 if empty)
GET	/api/history	Last 30 records, newest first
POST	/api/refresh	Manually trigger a scrape (admin token req.)
GET	/api/cron/scrape	Vercel Cron endpoint (requires CRON_SECRET auth)

Embeddable widget

A single-file JavaScript widget (public/widget.js) can be dropped into any website to display the current fuel price cap. No build step, no dependencies, Shadow-DOM-isolated so it won't clash with host-page CSS.

Quick start

Drop this anywhere in your HTML — the widget auto-mounts at the end of <body>:

<script src="https://zakolikdnes.cz/widget.js"></script>

Control theme and language:

<script src="https://zakolikdnes.cz/widget.js"
        data-theme="dark"
        data-lang="cs"></script>

Mount into a specific element instead of auto-appending:

<div id="fuel-cap-widget"></div>
<script src="https://zakolikdnes.cz/widget.js" data-theme="dark"></script>

Configuration

All attributes go on the <script> tag (or on the host element, which takes precedence — useful when the script is injected dynamically):

Attribute	Values	Default	Description
data-theme	light | dark	light	Colour scheme.
data-lang	en | cs	en	UI labels, number and date formatting.
data-target	CSS selector	—	Explicit mount target. Overrides #fuel-cap-widget.
data-api	URL	production	API base override (for local testing or self-hosting).

What it does

• Fetches /api/latest (3 s timeout) and renders Gasoline / Diesel / Updated in an isolated Shadow DOM.
• Adds a ↑ ↓ — trend indicator by diffing against /api/history[1] (best-effort; widget still renders if the history call fails).
• Animates the numbers on load (respects prefers-reduced-motion).
• Shows Data unavailable under the title on any fetch / validation failure.

Mount resolution order

1. data-target selector (on the script), if it resolves to one or more elements.
2. Every .fuel-cap-widget on the page (multi-instance).
3. #fuel-cap-widget, if present.
4. A fresh <div> appended to <body>.

Live preview

/embed renders all four theme × lang variants side-by-side with copy-paste snippets.

Tests

npm test

Covers the text-level extractor, validation rules (range, NaN, bad date) and a malformed input case that must not produce false positives.

Scheduler

Vercel Cron for /api/cron/scrape is disabled (empty crons in vercel.json) after the Ministry of Finance ended fuel price regulation on 19 July 2026. The scrape endpoint remains available for manual use via /api/refresh if needed.

Configuration

Var	Purpose
DATABASE_URL	Neon Postgres connection string
SOURCE_URL	Index page to discover bulletins
ADMIN_TOKEN	Header token for /api/refresh
CRON_SECRET	Vercel Cron authentication secret

Project layout

src/
  app/                Next.js App Router (UI + API routes)
  components/         PriceCard, PriceChart, RefreshButton
  lib/                config, db, date, http, logger
  scraper/            source discovery, PDF/HTML parsing, validation, orchestrator
scripts/
  migrate.ts          One-time schema bootstrap

Notes

• There is no official API for the bulletin — the scraper picks the newest "Cenový věstník" link from the configured index page, prefers the PDF, and falls back to HTML.
• Parsing uses regex/heuristics tolerant to whitespace and Czech diacritics. Validation enforces a 20–100 CZK/l range; out-of-range values are rejected and never written.
• Prices are de-duplicated by bulletin_id (primary key + ON CONFLICT DO NOTHING). effective_date has a unique constraint as well.