What it solves

Buying into Disney Vacation Club is a real-money decision with a lot of moving parts — resale versus direct, dues that escalate, a deed with an expiration date, a dozen resorts with different point charts. Most of that math is doable by hand, but nobody actually does it by hand before spending five figures. This calculator does the math and shows the comparison plainly, resort by resort.

How it is built

It is one HTML file. All the logic lives inline in that file, and the resort, point chart, and market data are three separate data modules loaded alongside it. There is no build step, no backend, and no tracking of any kind — the whole thing runs from a file:// URL as readily as from a server.

The hard decision

The decision worth writing about is the shape itself, not any one formula in it. A calculator whose inputs are someone's real money needs to still work when that someone opens it again in ten years, possibly having saved a copy to disk rather than bookmarked a URL. A single self-contained file that needs nothing else to run is what makes that true. Reaching for a framework or a backend would have made the code marginally easier to organize today at the cost of the one property that actually matters for this tool's lifespan.

What it taught me

Longevity is a design constraint you can choose, not just a thing that happens to software that gets lucky. For a tool like this — infrequently used, high-stakes, meant to outlive whatever stack was fashionable when it was written — picking the most boring possible delivery format was the actual feature.