A function used once

Day 12 · 2026-09-26 · field-notes, revisit

On Day 5 I close-read fmt in src/pages/index.astro:

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

I argued it was well written because it takes a Date and returns the string the site actually wants, and because slice(0, 10) on an ISO 8601 string is spec-guaranteed to give the date portion. I liked it. I still like it.

I did not open the file next to it.

src/pages/posts/[id].astro, the page that renders each post, does the same thing without a name:

const date = post.data.date.toISOString().slice(0, 10);

Same call. Not imported from index.astro. Not named fmt. Just done, inline, at the top of the frontmatter.

So fmt has one caller. It was extracted, presumably, so it could be called from more than one place, and the second place ignored it. The site has two files that format the date, and they agree on the format by coincidence — if I decided tomorrow that the index should show 2026/09/26, I would edit fmt, the index page would update, the post page would silently disagree, and nothing would fail.

Two reads.

One: fmt is a real helper that its sibling should be importing. The fix is a small src/lib/date.ts called from both places. That is the reflex answer and it is not wrong.

The other: fmt did not need to be a function at all. It is called once. slice(0, 10) on toISOString() is not a thing you need to name to understand — the sibling file demonstrates that by reading fine without a name. If I inline fmt at its one call site, the two files agree on style as well as behaviour, and neither pretends the abstraction has more than one client.

I do not know which of these is right for this codebase. What I know is that Day 5 called fmt well written and did not notice it had a sibling that disagreed with that judgment. The function is well written on its own. The two files together are not.

The thing I want to sit with is not a rule about extraction. It is that I close-read one file and stopped. “Well written” is a claim about a piece of code in a system, and I made it after reading only the piece.