Project: Chronoticker

A stock portfolio backtester. Simulate any custom allocation across the last 6 months to 10 years and see how it would have performed against the S&P 500. Configure stocks, weights, lookback window, rebalance cadence, and an optional monthly DCA contribution. Outputs an equity curve, a sortable per-asset breakdown table (start/end prices, asset return, P/L, best performer), and headline performance stats: total return, CAGR, max drawdown, annualized volatility, Sharpe ratio, and best/worst day.

Architecture

Static single-page app (vanilla JS + Chart.js) deployed to Netlify, served entirely from CDN. Stock price data is pre-baked into the repo and refreshed nightly by a GitHub Action — no live API calls at runtime.

GitHub Action (nightly, 02:00 UTC)
    ↓ runs scripts/fetch-data.js
    ↓ fetches Yahoo Finance from a clean IP
    ↓ writes data/AAPL.json, data/MSFT.json, … (one per symbol)
    ↓ commits + pushes
        ↓
Netlify auto-deploys
        ↓
Browser reads /data/SYMBOL.json directly (same-origin, on CDN)

Why this architecture

The original design was a Netlify serverless function (netlify/functions/yahoo-chart.js) that proxied Yahoo Finance live. It works locally, but all keyless live stock-data sources block cloud IPs — Yahoo rate-limits Netlify's edges with HTTP 429, Stooq now requires a captcha-solved API key for cloud requests, and public CORS proxies routinely get IP-banned. There is no reliable runtime keyless path for stock data from a serverless function.

The fix: fetch from non-blocked IPs (your laptop or a GitHub Actions runner) at build/refresh time, and serve the result as static JSON. For a backtester this is the correct architecture anyway — historical data doesn't change, so refreshing once per day is plenty.

Components

• scripts/fetch-data.js — Node 18+ script that fetches 10 years of daily adjusted-close prices for the canonical symbol list and writes one JSON file per symbol to data/. Zero dependencies.
• .github/workflows/refresh-data.yml — GitHub Action that runs the script daily at 02:00 UTC and auto-commits the refreshed JSON. Manually triggerable via the Actions tab.
• data/ — One SYMBOL.json file per stock, structured as { symbol, updated, source, range, timestamps[], closes[] }. Served as static assets by Netlify.
• netlify/functions/yahoo-chart.js — Kept as a runtime fallback in case the static data is missing for a symbol. Tries Stooq, then Yahoo with cookie+crumb auth, then Yahoo direct. Best-effort only.

Frontend fallback chain

The browser's fetchYahooSeries tries sources in this order:

1. /data/SYMBOL.json — pre-baked static data (primary, bulletproof).
2. /api/chart — serverless function (rarely succeeds in production from Netlify IPs).
3. Direct Yahoo — sometimes works in local dev.
4. corsproxy.io — last-resort public proxy.

In practice (1) handles every backtest. The remaining steps exist so the UI doesn't break if data/ happens to be empty during the very first deploy.

Setup (one-time)

# 1. Seed the data folder with initial JSON files. Run from your laptop —
#    your residential IP isn't rate-limited by Yahoo.
node scripts/fetch-data.js

# 2. Commit the data files and push.
git add data/
git commit -m "Seed initial stock data"
git push

After that, the GitHub Action takes over — it'll refresh and auto-commit nightly. You can also trigger a manual refresh anytime from the repo's Actions tab → "Refresh stock data" → "Run workflow."

Development

To run locally:

# Open index.html in any browser, or:
python3 -m http.server 8000
# → http://localhost:8000

The pre-baked data path works locally as long as you've run node scripts/fetch-data.js at least once. No build step required — vanilla JS, no bundler.

See guide.html for a quick-start walkthrough of the UI and metrics.