Price history tracker browser extension

A browser extension that logs prices on every product page you visit and alerts you when an item drops below its historical low, stored entirely locally.

The idea

A browser extension that silently records the price shown on any product page you visit — Amazon, Target, a niche shop, whatever — into a local SQLite database, then draws a small price-history sparkline next to the product title. When a tracked item drops below its recorded low, or below a threshold you set, it fires a browser notification. No account, no server, no price data leaving the machine.

Why build this

Shoppers already know retailers vary prices by session, device, and day, but there's no easy way to see the pattern without installing a heavyweight extension that phones prices home to a third-party service (and often takes a cut via affiliate-link injection). A local-only tracker sidesteps the privacy trade and the "is this tool secretly making money off my clicks" suspicion that surrounds most price trackers today. Manifest V3 content scripts and local storage APIs make this cheap to build now, and the shopping habits of anyone who bookmarks wishlists or waits for sales are a natural fit.

Stack sketch

Manifest V3 extension (Chrome + Firefox via webextension-polyfill), TypeScript
Per-site content scripts using simple CSS selector configs (JSON) to extract price + title + image; a handful of built-in configs for major retailers, user-editable for others
sql.js or the browser's native IndexedDB for local price history storage — no backend at all
Background service worker on an alarm (chrome.alarms) to re-check open/pinned tracked-product URLs periodically via a lightweight fetch + DOM parse, without needing the tab open
Native chrome.notifications for price-drop alerts
Small popup UI (Preact) rendering a sparkline (uPlot or hand-rolled canvas) per tracked item

Scope for v1

In: manual "track this page" button, automatic price logging on every visit to a previously tracked product, a popup list of tracked items with current price and all-time low, drop notifications
In: 5–8 built-in site configs (Amazon, Target, Walmart, Best Buy, Newegg) plus a generic fallback that asks the user to click the price element once to teach it the selector
Out: cross-device sync, browser-side scheduled re-checks for closed tabs (v1 only updates prices on visit; background polling is a v1.1 stretch once the core loop is proven), affiliate links, multi-currency conversion, price prediction

Where it could go

The natural next step is opt-in background polling for tracked-but-closed tabs, so the extension actually finds drops instead of just recording ones you happen to see — this is the feature users will ask for first, and it's the one that requires the most care (rate limiting requests per domain, respecting robots.txt-style etiquette, handling anti-bot pages gracefully). After that, an optional encrypted sync via something like a user-supplied WebDAV or S3 bucket would let people track across a laptop and phone without standing up an account system. A "share my watchlist" export (a flat JSON or CSV) is a low-effort way to let people compare notes without building social features.

Watch out for

Scraping product pages for personal use is generally fine, but background polling of closed tabs edges toward automated fetching of retailer pages, which some sites' terms of service prohibit — keep polling opt-in, low-frequency, and clearly scoped to pages the user explicitly tracked, and be ready for site layout changes to silently break a config (a "this price stopped updating, click to fix the selector" prompt beats silent failure).