A builder, and why TypeScript mostly uses options objects

// hold.tsexport interface Hold { seatId: string; status: 'held' | 'expired'; expiresAt: number }export const releasesSeat = (h: Hold) => h.status === 'expired'; // hold-builder.ts (version 1)import type { Hold } from './hold';export class HoldBuilder {  private hold: Hold = { seatId: 'A1', status: 'held', expiresAt: 600 };  forSeat(seatId: string) { this.hold.seatId = seatId; return this; }  expired() { this.hold.status = 'expired'; return this; }  build(): Hold { return JSON.parse(JSON.stringify(this.hold)); }} // hold-builder.ts (version 2)import type { Hold } from './hold';export class HoldBuilder {  constructor(private hold: Hold = { seatId: 'A1', status: 'held', expiresAt: 600 }) {}  forSeat(seatId: string) { return new HoldBuilder({ ...this.hold, seatId }); }  expired() { return new HoldBuilder({ ...this.hold, status: 'expired' }); }  build(): Hold { return { ...this.hold }; }} // hold-builder.ts (version 3)import type { Hold } from './hold';export class HoldBuilder {  constructor(private hold: Hold = { seatId: 'A1', status: 'held', expiresAt: 600 }) {}  forSeat(seatId: string) { const next = new HoldBuilder(this.hold); next.hold.seatId = seatId; return next; }  expired() { const next = new HoldBuilder(this.hold); next.hold.status = 'expired'; return next; }  build(): Hold { return { ...this.hold }; }} // main.tsimport { HoldBuilder } from './hold-builder';import { releasesSeat } from './hold';const base = new HoldBuilder().forSeat('B7');const lapsed = base.expired().build();const live = base.build();console.log(`lapsed hold frees B7: ${releasesSeat(lapsed)} · live hold frees B7: ${releasesSeat(live)}`);

// holds-query.tsimport { db } from './db';export function holdsToRelease(eventId: string | undefined, now: number) {  let q = db.selectFrom('holds').select('seat_id').where('status', '=', 'held');  if (eventId) q.where('event_id', '=', eventId);  q = q.where('expires_at', '<', now);  return q.compile().sql;} // main.tsimport { holdsToRelease } from './holds-query';console.log(holdsToRelease('EVT-9', 600));

// order.tsexport class InvalidOrder extends Error {}export interface Order { seats: number; maxSeats: number } // order-builder.ts (version 1)import { InvalidOrder, type Order } from './order';export class OrderBuilder {  private seats = 1;  private maxSeats = 4;  withSeats(n: number) { this.seats = n; return this; }  forGroup() { this.maxSeats = 10; if (this.seats > this.maxSeats) throw new InvalidOrder(); return this; }  build(): Order { return { seats: this.seats, maxSeats: this.maxSeats }; }} // order-builder.ts (version 2)import { InvalidOrder, type Order } from './order';export class OrderBuilder {  private seats = 1;  private maxSeats = 4;  withSeats(n: number) { if (n > this.maxSeats) throw new InvalidOrder(); this.seats = n; return this; }  forGroup() { this.maxSeats = 10; return this; }  build(): Order { return { seats: this.seats, maxSeats: this.maxSeats }; }} // order-builder.ts (version 3)import { InvalidOrder, type Order } from './order';export class OrderBuilder {  private seats = 1;  private maxSeats = 4;  withSeats(n: number) { this.seats = n; return this; }  forGroup() { this.maxSeats = 10; return this; }  build(): Order { if (this.seats > this.maxSeats) throw new InvalidOrder(); return { seats: this.seats, maxSeats: this.maxSeats }; }} // main.tsimport { OrderBuilder } from './order-builder';const attempt = (label: string, make: () => unknown) => {  try { make(); console.log(`${label}: built`); } catch (e) { console.log(`${label}: ${(e as Error).constructor.name}`); }};attempt('group, 6 seats', () => new OrderBuilder().withSeats(6).forGroup().build());attempt('one buyer, 6 seats', () => new OrderBuilder().withSeats(6).build());

// hold.tsexport interface Hold { seatId: string; priceCents: number; expiresAt: number }export const reminderAt = (h: Hold) => h.expiresAt - 60;export const priceLabel = (h: Hold) => `€${(h.priceCents / 100).toFixed(2)}`; // hold-builder.ts (after the MR)import type { Hold } from './hold';const DEFAULTS: Hold = { seatId: 'A1', priceCents: 4000, expiresAt: 300 }; // was 600export class HoldBuilder {  constructor(private readonly fields: Hold = DEFAULTS) {}  withExpiry(expiresAt: number) { return new HoldBuilder({ ...this.fields, expiresAt }); }  withPrice(priceCents: number) { return new HoldBuilder({ ...this.fields, priceCents }); }  build(): Hold { return { ...this.fields }; }}export const aHold = () => new HoldBuilder(); // main.tsimport { aHold } from './hold-builder';import { reminderAt, priceLabel } from './hold';const check = (name: string, ok: boolean) => console.log(`${ok ? 'pass' : 'FAIL'} ${name}`);check('reminder a minute before expiry', reminderAt(aHold().build()) === 540);check('reminder for a 15-minute hold', reminderAt(aHold().withExpiry(900).build()) === 840);check('price label', priceLabel(aHold().withPrice(1250).build()) === '€12.50');

// transfer.tsexport interface Transfer { from: string; to: string; amountCents: number; feeCents: number }export function transfer(from: string, to: string, amountCents: number, feeCents: number): Transfer {  return { from, to, amountCents, feeCents };}export function transferOf(t: Transfer): Transfer {  return { ...t };} // main.tsimport { transfer, transferOf } from './transfer';const feeCents = 250;const amountCents = 10000;const a = transfer('acc-1', 'acc-2', feeCents, amountCents);const b = transferOf({ from: 'acc-1', to: 'acc-2', feeCents, amountCents });console.log(`positional: ${a.amountCents} · options: ${b.amountCents}`);

// message-builder.tsexport interface Message { room: string; to: string; text: string; silent: boolean }export class MessageBuilder {  constructor(private m: Message = { room: 'general', to: '', text: '', silent: false }) {}  inRoom(room: string) { return new MessageBuilder({ ...this.m, room }); }  to(user: string) { return new MessageBuilder({ ...this.m, to: user }); }  saying(text: string) { return new MessageBuilder({ ...this.m, text }); }  silently() { this.m.silent = true; return this; }  build(): Message { return { ...this.m }; }} // main.tsimport { MessageBuilder } from './message-builder';const users = [{ id: 'ana', muted: false }, { id: 'ben', muted: true }, { id: 'cy', muted: false }];const b = new MessageBuilder().inRoom('launch').saying('Deploy starts at 14:00');const sent = users.map((u) => { if (u.muted) b.silently(); return b.to(u.id).build(); });console.log(sent.map((m) => `${m.to}:${m.silent ? 'silent' : 'ping'}`).join(' '));