Factory, abstract factory and NestJS useFactory

// drm.tsexport interface Player { drm: string; }export interface LicenceClient { drm: string; }export interface DrmKit { player(): Player; licence(): LicenceClient; } export class WidevinePlayer implements Player { drm = 'widevine'; }export class FairPlayPlayer implements Player { drm = 'fairplay'; }export class WidevineLicence implements LicenceClient { drm = 'widevine'; }export class FairPlayLicence implements LicenceClient { drm = 'fairplay'; } export const widevineKit: DrmKit = { player: () => new WidevinePlayer(), licence: () => new WidevineLicence() };export const fairplayKit: DrmKit = { player: () => new FairPlayPlayer(), licence: () => new FairPlayLicence() };

// signing.tsimport { Injectable } from '@nestjs/common'; export interface UrlSigner { sign(titleId: string): string; }export class NorthSigner implements UrlSigner { sign(titleId: string) { return `north/${titleId}`; } }export class SkySigner implements UrlSigner { sign(titleId: string) { return `sky/${titleId}`; } } export const signerFor = (cdn: string): UrlSigner => (cdn === 'sky' ? new SkySigner() : new NorthSigner()); @Injectable()export class StudioContracts {  cdnFor(studio: string) { return studio === 'studio-b' ? 'sky' : 'north'; }}

// signing.tsexport interface UrlSigner { sign(titleId: string): string; }export class NorthSigner implements UrlSigner { sign(titleId: string) { return `north/${titleId}`; } }export class SkySigner implements UrlSigner { sign(titleId: string) { return `sky/${titleId}`; } } export function signerFor(cdn: string): UrlSigner {  return cdn === 'sky' ? new SkySigner() : new NorthSigner();} // playback.module.tsimport { Inject, Injectable, Module } from '@nestjs/common';import { signerFor, UrlSigner } from './signing.js'; export const SIGNER = Symbol('SIGNER'); @Injectable()export class CdnConfig { cdn() { return 'sky'; } } @Injectable()export class PlaybackService {  constructor(@Inject(SIGNER) private signer: UrlSigner) {}  play(titleId: string) { return this.signer.sign(titleId); }} @Module({  providers: [CdnConfig, PlaybackService, { provide: SIGNER, inject: [CdnConfig], useFactory: (config: CdnConfig) => signerFor(config.cdn()) }],})export class PlaybackModule {} // playback.a.spec.tsimport assert from 'node:assert/strict';import { Test } from '@nestjs/testing';import { PlaybackModule, PlaybackService, SIGNER } from './playback.module.js'; export async function run() {  const moduleRef = await Test.createTestingModule({ imports: [PlaybackModule] })    .overrideProvider(SIGNER)    .useValue({ sign: (titleId: string) => `sky/${titleId}` })    .compile();  assert.equal(moduleRef.get(PlaybackService).play('t-1'), 'sky/t-1');} // signing.b.spec.tsimport assert from 'node:assert/strict';import { SkySigner, signerFor } from './signing.js'; export async function run() {  assert.ok(signerFor('sky') instanceof SkySigner);} // playback.c.spec.tsimport assert from 'node:assert/strict';import { Test } from '@nestjs/testing';import { PlaybackModule, PlaybackService } from './playback.module.js'; export async function run() {  const moduleRef = await Test.createTestingModule({ imports: [PlaybackModule] }).compile();  assert.match(moduleRef.get(PlaybackService).play('t-1'), /^\w+\/t-1$/);}

// signing.tsexport interface UrlSigner { sign(titleId: string): string; } export class NorthSigner implements UrlSigner {  sign(titleId: string) { return `north/${titleId}`; }  purge(titleId: string) { return `purged ${titleId}`; }} export class SkySigner implements UrlSigner {  sign(titleId: string) { return `sky/${titleId}`; }} export function signerFor(cdn: string): UrlSigner {  return cdn === 'north' ? new NorthSigner() : new SkySigner();} // takedown.tsimport { signerFor } from './signing.js'; const signer = signerFor('north');export const purged = signer.purge('t-9');

// gates.tsexport interface TicketPrinter { format: string; }export interface GateScanner { reads: string; } export class QrPrinter implements TicketPrinter { format = 'qr'; }export class NfcPrinter implements TicketPrinter { format = 'nfc'; }export class QrScanner implements GateScanner { reads = 'qr'; }export class NfcScanner implements GateScanner { reads = 'nfc'; } export const normalise = (gates: string) => gates.trim().toLowerCase();export const printerFor = (gates: string): TicketPrinter => (gates === 'nfc' ? new NfcPrinter() : new QrPrinter());export const scannerFor = (gates: string): GateScanner => (gates === 'nfc' ? new NfcScanner() : new QrScanner()); // setup.tsimport { normalise, printerFor, scannerFor } from './gates.js'; const contracts: Record<string, string> = { arena: 'nfc', theatre: 'QR', stadium: 'NFC', expo: 'nfc ' };export const agree = Object.values(contracts).filter((gates) => {  const scanner = scannerFor(normalise(gates));  const printer = printerFor(gates);  return printer.format === scanner.reads;}).length;

import { Inject, Injectable, Module } from '@nestjs/common'; export interface FxSource { rate(currency: string): number; }export class EcbRates implements FxSource { rate(currency: string) { return currency === 'EUR' ? 0.92 : 1; } }export class OandaRates implements FxSource { rate(currency: string) { return currency === 'EUR' ? 0.93 : 1; } } export class ContractStoreDown extends Error {} @Injectable()export class LedgerContracts {  async fxProvider(): Promise<string> { throw new ContractStoreDown('contract store unreachable'); }} export const FX_SOURCE = 'FX_SOURCE'; @Injectable()export class SettlementService {  constructor(@Inject(FX_SOURCE) private fx: FxSource) {}  settle(cents: number) { return Math.round(cents * this.fx.rate('EUR')); }} @Module({  providers: [LedgerContracts, SettlementService, {    provide: FX_SOURCE,    inject: [LedgerContracts],    useFactory: async (contracts: LedgerContracts) => ((await contracts.fxProvider()) === 'oanda' ? new OandaRates() : new EcbRates()),  }],})export class AppModule {}