TDD: red, green, refactor

A chat app's unread badge is blank at 0 and shows the count otherwise. Its code and two tests pass:

test('no badge at 0', () => assert.equal(unreadBadge(0), ''));
test('shows the count', () => assert.equal(unreadBadge(7), '7'));

The last two cases on the pair's list are that a badge above 99 reads '99+' and that 99 itself still reads '99'. They turned both into tests, one at a time, and the log of those steps lost its order.

A ticketing site refunds a cancelled booking in full 7 or more days before the show, half from 2 to 6 days before, and nothing after that. A pair has this code and two green tests:

test('full refund a week ahead', () => assert.equal(refundFor(80, 10), 80));
test('no refund the day before', () => assert.equal(refundFor(80, 1), 0));

Three cases are left on their list.

A payments ledger's transfer fee has two green tests, and the pair has just added a third, which fails:

test('2% of a large transfer', () => assert.equal(fee(10000), 200));
test('no fee on nothing', () => assert.equal(fee(0), 0));
test('a 30-cent minimum', () => assert.equal(fee(500), 30));

A pair starts test-driving a video player's "Skip intro" button. Before any code exists, the navigator types the first test:

the interviewer says: "ExamHall's multi-select questions need a scorer. Write multiSelectScore(chosen, correct): each chosen option that is correct earns 1, each chosen option that is not correct costs 1, a question never scores below 0, and an option chosen twice counts once. Don't change the arrays you're given. Let's pair: test-first." You have 20 minutes, a plain editor and node --test.

Before you type: say your test list out loud, each case with its input and what it expects.

the interviewer says: "ExamHall's multi-select questions need a scorer. Write multiSelectScore(chosen, correct): each chosen option that is correct earns 1, each chosen option that is not correct costs 1, a question never scores below 0, and an option chosen twice counts once. Don't change the arrays you're given. Let's pair: test-first." You have 20 minutes, a plain editor and node --test.

A chat app's typing line passes these five tests:

With all five green, the pair refactors the function to this:

export function typingLabel(names) {
  const [first, ...rest] = names;
  if (!first) return '';
  if (rest.length === 0) return `${first} is typing`;
  if (rest.length === 1) return `${first} and ${rest[0]} are typing`;
  return `${first} and ${rest.length - 1} others are typing`;
}

They run the five tests.

A video app makes one preview thumbnail per full 10 seconds of a clip, and never fewer than 1. The pair's first test passed on a hard-coded 6. Their second test, the two-minute clip, was red; while making it pass, the driver noticed that a clip shorter than 10 seconds would get 0, and wrote the minimum of 1 into the same change. A third case from their list followed, and all three tests pass:

test('a one-minute clip has 6 thumbnails', () => assert.equal(thumbnailCount(60), 6));
test('a two-minute clip has 12 thumbnails', () => assert.equal(thumbnailCount(120), 12));
test('a short clip has 3 thumbnails', () => assert.equal(thumbnailCount(30), 3));

A month later, a tidy-up turns the body back into Math.floor(seconds / 10).