Extending an existing codebase live

Pair round on a streaming service. The task: “Titles now carry a regions list. A title must not play for a viewer whose country is not in it.” playback.mjs:

The web app plays through routes.mjs, which answers a PlaybackDenied with 403 and its reason. The TV app's Continue watching row calls startPlayback directly. browse.mjs builds the catalogue a viewer scrolls.

export const ageGuard = (viewer, title) => (title.minAge > viewer.age ? 'age' : null);export const planGuard = (viewer, title) => (title.premium && viewer.plan !== 'premium' ? 'plan' : null);export const GUARDS = [ageGuard, planGuard]; export function startPlayback(ctx, viewer, titleId) {  const title = ctx.catalog.get(titleId);  for (const guard of GUARDS) {    const reason = guard(viewer, title);    if (reason) throw new PlaybackDenied(reason);  }  ctx.streams.push({ viewer: viewer.id, titleId });  return { titleId, from: viewer.positions[titleId] ?? 0 };}

MR !3451 on the same streaming service. The task: “Add skipIntroAt (seconds, or null if the episode has no intro) to GET /episodes/:id.” The service has 13 source files. The candidate's diff touches two of them, and the episode tests she ran pass:

 // episodes.mjs
-export function getEpisode(ctx, id) { return ctx.episodes.get(id); }
+export function fetchEpisode(ctx, id) { return ctx.episodes.get(id); }
 export function fetchSeries(ctx, id) { return ctx.series.get(id); }

 export function runtime(ep) {
-  let total = 0;
-  for (const part of ep.parts) total += part.seconds;
-  return total;
+  return ep.parts.reduce((total, part) => total + part.seconds, 0);
 }

 export function toEpisodeDto(ep) {
-  return { id: ep.id, title: ep.title, seconds: runtime(ep) };
+  return { id: ep.id, title: ep.title, seconds: runtime(ep), skipIntroAt: ep.introEnd ?? null };
 }
 // episode.controller.mjs
-  return episodes.toEpisodeDto(episodes.getEpisode(ctx, id));
+  return episodes.toEpisodeDto(episodes.fetchEpisode(ctx, id));

Before the review, a search of the service for getEpisode( printed:

$ grep -n "getEpisode(" *.mjs
downloads.mjs:3:  const ep = episodes.getEpisode(ctx, id);
episode.controller.mjs:3:  return episodes.toEpisodeDto(episodes.getEpisode(ctx, id));
episodes.mjs:2:export function getEpisode(ctx, id) { return ctx.episodes.get(id); }

The episode test checks the reply's seconds, and the TV app reads the reply with const { id, title, seconds } = JSON.parse(json);. Run against the whole service's tests, one of the three changes turns a test red.

Back on the ExamHall service, her extendCandidate is in and every test passes; the interviewer has asked for nothing more. She will name one improvement to the existing code as a follow-up, with its reason, and change nothing. The timer module:

Two of the service's methods:

  extendAll(examId, minutes, by) {
    for (const c of this.store.get(examId).values()) c.timer = this.timer.addTime(c.timer, minutes * 60_000);
    this.audit.push({ type: 'exam.extended', examId, minutes, by });
  }

  pauseCandidate(examId, candidateId, by) {
    const c = this.candidate(examId, candidateId);
    c.timer = this.timer.pause(c.timer, this.clock.now());
    this.audit.push({ type: 'candidate.paused', examId, candidateId, by });
  }

The controller turns a NotFound into 404 and any other error into 500.

export function startTimer(now, durationMs) { return { endsAt: now + durationMs, pausedAt: null }; }export function remaining(t, now) { return Math.max(0, t.endsAt - (t.pausedAt ?? now)); }export function addTime(t, ms) { return { ...t, endsAt: t.endsAt + ms }; }export function pause(t, now) { return t.pausedAt === null ? { ...t, pausedAt: now } : t; }export function resume(t, now) { return t.pausedAt === null ? t : { endsAt: t.endsAt + (now - t.pausedAt), pausedAt: null }; }

Later in the same round, with extendCandidate in, the interviewer adds: “Fire alarms: let a proctor pause the whole exam with one request.” They will call POST /exams/econ-201/pause at minute 30, then check that at minute 40 every candidate still has the time left they had at 30, and that the audit trail gained one entry, of type exam.paused. The routes:

The service's methods after candidate:

  extendAll(examId, minutes, by) {
    for (const c of this.store.get(examId).values()) c.timer = this.timer.addTime(c.timer, minutes * 60_000);
    this.audit.push({ type: 'exam.extended', examId, minutes, by });
  }

  pauseCandidate(examId, candidateId, by) {
    const c = this.candidate(examId, candidateId);
    c.timer = this.timer.pause(c.timer, this.clock.now());
    this.audit.push({ type: 'candidate.paused', examId, candidateId, by });
  }

  extendCandidate(examId, candidateId, minutes, by) {
    const c = this.candidate(examId, candidateId);
    c.timer = this.timer.addTime(c.timer, minutes * 60_000);
    this.audit.push({ type: 'candidate.extended', examId, candidateId, minutes, by });
  }

The timer module:

export function startTimer(now, durationMs) { return { endsAt: now + durationMs, pausedAt: null }; }
export function remaining(t, now) { return Math.max(0, t.endsAt - (t.pausedAt ?? now)); }
export function addTime(t, ms) { return { ...t, endsAt: t.endsAt + ms }; }
export function pause(t, now) { return t.pausedAt === null ? { ...t, pausedAt: now } : t; }
export function resume(t, now) { return t.pausedAt === null ? t : { endsAt: t.endsAt + (now - t.pausedAt), pausedAt: null }; }

The proctor dashboard's tile, countdownLabel, shows paused for a candidate whose timer has pausedAt set. Before typing, she picks the existing code her feature will follow.

    'POST /exams/:examId/extend': (p, body, user) => svc.extendAll(p.examId, body.minutes, user),    'POST /exams/:examId/candidates/:candidateId/pause': (p, body, user) => svc.pauseCandidate(p.examId, p.candidateId, user),    'POST /exams/:examId/candidates/:candidateId/extend': (p, body, user) => svc.extendCandidate(p.examId, p.candidateId, body.minutes, user),

Work in a copy of the ExamHall fixture.

On the streaming service, the task is to show an episode card's running time as 1:05 instead of 1 h 5 min. The candidate changes the shared helper's default, compact = false, to compact = true:

Four screens use format.mjs:

export const cardLine = (format, ep) => `${ep.title} · ${format.formatDuration(ep.seconds)}`;
export const overlayLine = (format, left) => `${format.formatDuration(left, { compact: true })} left`;
export const resultLine = (format, ep) => `${ep.title} (${format.formatRuntime(ep)})`;
export const downloadLine = (format, d) => `${d.title} · ${format.formatDuration(d.seconds, { compact: false })} · ${d.mb} MB`;

// format.mjsexport function formatDuration(seconds, { compact = true } = {}) {  const h = Math.floor(seconds / 3600), m = Math.floor((seconds % 3600) / 60);  if (compact) return `${h}:${String(m).padStart(2, '0')}`;  return h ? `${h} h ${m} min` : `${m} min`;}export function formatRuntime(ep) { return formatDuration(ep.seconds); }

A learner aiming at 2026 interviews at a video-streaming company practises only by solving puzzles in blank files.