Refactoring safely

The catalog package exports discounted, which other teams' code imports:

// pricing/discount.test.mjs
import { test } from 'node:test';
import assert from 'node:assert/strict';
import { discounted } from './index.mjs';

test('10% off', () => assert.equal(discounted(1000, 10), 900));
test('15% off', () => assert.equal(discounted(999, 15), 849));
test('100% off', () => assert.equal(discounted(500, 100), 0));
test('nothing off', () => assert.equal(discounted(1200, 0), 1200));
test('33% off', () => assert.equal(discounted(250, 33), 168));

An MR titled "refactor: tidy discounted()" has four commits, and the five tests passed after each:

1. if (p < 0) p = 0; becomes p = Math.max(p, 0);
2. the first line becomes export function discounted(pricePence, percentOff = 0) {
3. the function moves, unchanged, into a new pricing/discount.mjs, and pricing/index.mjs becomes export { discounted } from './discount.mjs';
4. in the test file, '10% off' becomes 'ten percent off a 1000p item'

Ola is refactoring a streaming app's watchTimeLabel in small steps, running its six tests after each. Her last commit holds the code from before step 1:

// player/watch-time.test.mjs
import { test } from 'node:test';
import assert from 'node:assert/strict';
import { watchTimeLabel } from './watch-time.mjs';

test('0 s', () => assert.equal(watchTimeLabel(0), '0 min'));
test('90 s', () => assert.equal(watchTimeLabel(90), '1 min'));
test('600 s', () => assert.equal(watchTimeLabel(600), '10 min'));
test('3599 s', () => assert.equal(watchTimeLabel(3599), '59 min'));
test('3660 s', () => assert.equal(watchTimeLabel(3660), '1 h 01 min'));
test('7325 s', () => assert.equal(watchTimeLabel(7325), '2 h 02 min'));

The tests passed after each of steps 1–3, which she has not committed. After step 3 the file reads:

// player/watch-time.mjs (after step 3)
const pad2 = (n) => String(n).padStart(2, '0');

export function watchTimeLabel(seconds) {
  const totalMinutes = Math.floor(seconds / 60);
  const hours = Math.floor(totalMinutes / 60);
  const minutes = totalMinutes % 60;
  if (hours > 0) return `${hours} h ${pad2(minutes)} min`;
  return `${minutes} min`;
}

Step 4 replaces the three const lines with two:

  const hours = Math.floor(seconds / 3600);
  const minutes = Math.round((seconds % 3600) / 60);

Now the 90 s and 3599 s tests fail.

A chat app's renderMessage(text) walks the text once, character by character, escaping HTML and turning @mentions into links in the same loop, and its 14 tests pass. Product now wants :shortcode: emoji, and every way Kai tries to fit them into the loop breaks the mention handling.

Sam is moving a ticket shop's booking-fee code out of checkout.mjs into a module of its own, one small step at a time. Halfway through, he sees that the fee on a £13.10 ticket, 2.5% or 32.75p, is rounded down to 32p, where the box office's rule says the nearest penny.

A box office's seat holds. git log --oneline on main, newest first:

7465d89 feat: show a countdown on held seats
10fc1a5 refactor: tidy holdExpiresAt (also holds 7 -> 10 minutes)
8cba506 holds: expire a seat hold after 7 minutes

The middle commit named the constants, changed the hold from 7 to 10 minutes and moved the two hold tests from 420 to 600 seconds. The newest one exported HOLD_MINUTES, added secondsLeft just below holdExpiresAt, and a seat-map countdown that imports both. holds/hold-expiry.mjs on main now reads:

The box office now wants 7-minute holds back. The countdown and the tidied code must stay, and every test must pass.

A ledger's statement total:

Finance wants a loyalty credit taken off each statement, and a statement must never show less than 0. Pia plans two commits. The second will change only the body of a new function, afterCredits(pence, statement), to return pence - statement.creditPence;.