Testing as you go

Before a designer publishes a jumper pattern, a test knitter checks it. She knits the sample size and it comes out right. Before she calls the pattern ready, she picks the sizes that differ: the smallest, the largest, and the one whose lace panel has no full repeat. With no time to knit them all, she counts stitches row by row on paper: what each row says, not the shape the designer meant. When the largest size comes out two stitches short, she tells the designer, the row is fixed, and she counts it again.

With the pattern	In the round
The sample size knits up right	The interviewer's example passes
The smallest, the largest, the size with no full repeat	Named cases: the limits and empty
Choosing those sizes before calling it ready	Naming cases before "done"
Counting stitches row by row on paper	A hand trace in a plain editor
Counting what each row says, not the intended shape	Tracing the lines, not the intent
Two stitches short: told, fixed, counted again	A caught bug said aloud, fixed, re-checked
"Ready?" answered with the sizes checked	"Confident?" answered with the cases checked

Breaks down when: a pattern has a fixed list of sizes, while code meets inputs nobody listed. Wrong inference: "if my cases pass, every input is safe." The designer also reads her full report later, while an interviewer sees only the checks you show during the round.

In real terms: before "done", say three cases and check each one, out loud.

4.1 ExamHall EXH-3140: the third named case goes red

In a practice pair round, the candidate built the AnswerEditor's autosave: input(text) saves the latest text 800 ms after the last key, and the editor calls dispose() when it closes. The interviewer's example passed. Before saying done, she named three cases: rapid typing, a single key, and dispose before the save fires.

4.1 EXH-3140 draft · the interviewer's example (keys at 0, 200, 400 ms): saves "Dem" at 1200 ms · pass
4.1 EXH-3140 draft · named cases: 2 of 3 pass · red: dispose before it fires (saved "D" at 800 ms, 300 ms after dispose at 500 ms)
4.1 MR !3440 dispose clears the scheduled save · named cases: 3 of 3 pass

The example could never have shown this: nothing in it calls dispose(). The draft's dispose() set its timer variable to null without clearing the scheduled save (steps 1–3). She said “dispose doesn't cancel the save”, fixed it, and ran the three cases again (step 8). How a debounced save is written belongs to kata-debounce.

4.2 Chat, MR !3441: a hand trace of trimHistory

export function trimHistory(messages, max) {
  return messages.slice(-max);
}

The chat app keeps the last max messages of a thread. With no runner, the candidate traces the case max = 0, the limit, on a thread of 5:

4.2 trimHistory(thread of 5, 2) · slice(-2) · 2 messages kept · spec 2 · pass
4.2 trimHistory(thread of 5, 0) traced line by line
4.2   max = 0 · -max = -0
4.2   messages.slice(-0) is messages.slice(0) · 5 messages kept
4.2   returned 5 messages · spec 0 · red
4.2 MR !3441 (return [] when max <= 0) · max 0: 0 kept · max 2: 2 kept · pass

Read as intent, slice(-max) "takes the last none". Traced line by line, -0 is 0, and slice(0) keeps everything (steps 6–7).