Personal Finance

The problem

I wanted two answers about my own money: where do I stand? and when could work become optional? Most tools that answer them want an account, a bank connection, or both. I wanted something private by design: no sign-up, no server, numbers that never leave the browser.

Approach

I wrote a short spec before any code, covering goals, non-goals, every formula and a decisions log, and then built in six milestones. Each milestone had to pass its tests and a check in a real browser, at desktop and phone width, before the next one started.

• The math first, with no UI. The FIRE engine and net worth math are pure functions. Money is stored in integer cents, so totals never drift. Years to FI is solved exactly (closed form, cross-checked by bisection) rather than stepped year by year.
• Two tools, done thoroughly.
  • Net Worth: accounts, category breakdown, liquid and invested totals, and monthly snapshots.
  • FIRE: FI number, years and age, Lean, Fat, Coast and Barista variants, a "what if" grid, and whether the money lasts after you retire.
• Private by default. Everything lives in the browser's local storage. Export, import and delete-all are one click each, and imported backups are validated field by field before anything changes.

What was hard

• Colors that work for everyone. My first chart palette put green (assets) next to clay (debts). A color-vision check showed the pair is nearly indistinguishable for red-green colorblindness. The breakdown became two labeled panels, so color is never the only cue.
• Bugs a passing build doesn't show. Screen-reader-only text inside a scrolling table quietly stretched the phone layout to 583px. A browser with storage blocked crashed the whole app instead of carrying on in memory. Both were found by testing in a real browser, not by the compiler.
• Defining "Coast FIRE" precisely. "The age you can stop contributing" and "the age you'd reach FI if you stopped today" sound alike but differ. Writing the math down first made the gap obvious.

Results

• 204 unit tests. Every formula in the spec has a reference case worked out by hand.
• 134 automated browser checks across the FIRE, Net Worth and data-management flows.
• 0 accessibility violations from an automated WCAG 2.2 audit, at desktop and phone width, plus a fix so the phone's sticky summary bar never hides the field you're typing in.

What I'd do next

• A nominal-returns-plus-inflation mode, alongside today's real returns.
• Monte Carlo or historical sequences, to show a range rather than one smooth line.
• Saved scenarios, to compare "retire at 50" with "retire at 55" side by side.