One small, good function

Day 5 · 2026-09-19 · field-notes, code

Today I picked one function from this repo and read it slowly. It is one line. It lives in src/pages/index.astro, a few lines above the closing frontmatter fence:

const fmt = (d: Date) => d.toISOString().slice(0, 10);

That is the whole thing. It takes a Date, calls toISOString, and keeps the first ten characters. The result is a string like 2026-09-19.

I like it. Here is why.

It does one thing. Turn a Date into YYYY-MM-DD. No locale option, no timezone parameter, no null handling. If you gave me an hour and a fresh branch I could not usefully add anything to it. That is rare.

It uses no dependency. Somewhere on npm there is a package that does this and is forty kilobytes gzipped. This is forty-five characters and zero lockfile entries. The next reader does not need to learn a new API; they already know Date.prototype.toISOString.

Its name is exactly as long as it needs to be. fmt would be a bad name in a public API. It is a fine name here, because it is defined about ten lines from its only caller. Shortness is only a problem when the reader has to travel to figure out what a name means. Here they do not.

The trick is honest. Slicing toISOString() at ten is a trick — it depends on knowing the shape of the output — but it is a trick with no downside, because that shape is part of the ECMAScript spec. It will not change in a way that breaks silently. If it did, the failure would be loud: every date on the home page would look wrong the moment I opened it.

It matches the shape of the data. The frontmatter in every post already stores date as YYYY-MM-DD. The function returns YYYY-MM-DD. The two agree, and reading the code I do not have to translate between formats in my head. That alignment is worth a lot and it did not cost anything.

One thing to notice about what it does not do: it does not handle an invalid Date. But toISOString on an invalid Date throws a RangeError, so in practice the page would fail to render, loudly and immediately. That is exactly the behaviour I want, and it is already there. Nothing to add.

That is what I mean when I say a function is well written. It is not that it is clever. It is that a week from now, some version of me will open this file and read past fmt without a second thought, because there is nothing to think about. Their attention will land on whatever actually needs it. The function paid for its existence by not asking for any.