feat(): durable providers #2 - i18n

Author: egocentryk

package.json

@@ -26,9 +26,11 @@
     "@nestjs/event-emitter": "^3.0.1",
     "@nestjs/mapped-types": "*",
     "@nestjs/platform-express": "^11.0.1",
+    "accept-language-parser": "^1.5.0",
     "piscina": "5.0.0-alpha.0",
     "reflect-metadata": "^0.2.2",
-    "rxjs": "^7.8.1"
+    "rxjs": "^7.8.1",
+    "string-format": "^2.0.0"
   },
   "devDependencies": {
     "@eslint/eslintrc": "^3.2.0",
@@ -38,9 +40,11 @@
     "@nestjs/testing": "^11.0.1",
     "@swc/cli": "^0.6.0",
     "@swc/core": "^1.10.7",
+    "@types/accept-language-parser": "^1.5.7",
     "@types/express": "^5.0.0",
     "@types/jest": "^29.5.14",
     "@types/node": "^22.10.7",
+    "@types/string-format": "^2.0.3",
     "@types/supertest": "^6.0.2",
     "eslint": "^9.18.0",
     "eslint-config-prettier": "^10.0.1",

pnpm-lock.yaml

@@ -13,9 +13,10 @@ import { PaymentsModule } from './payments/payments.module'
 import { DataSourceModule } from './data-source/data-source.module'
 import { UsersModule } from './users/users.module'
 import { ContextIdFactory } from '@nestjs/core'
-import { AggregateByTenantContextIdStrategy } from './core/aggregate-by-tenant.strategy'
+import { I18nModule } from './i18n/i18n.module'
+import { AggregateByLocaleContextIdStrategy } from './core/aggregate-by-locale.strategy'
 
-ContextIdFactory.apply(new AggregateByTenantContextIdStrategy())
+ContextIdFactory.apply(new AggregateByLocaleContextIdStrategy())
 
 @Module({
   imports: [
@@ -37,6 +38,7 @@ ContextIdFactory.apply(new AggregateByTenantContextIdStrategy())
     PaymentsModule,
     DataSourceModule,
     UsersModule,
+    I18nModule,
     // Alternatively, we can use the `forRootAsync` method to register the module with dynamic options
     // HttpClientModule.forRootAsync({
     //   useFactory: () => ({ baseUrl: 'https://nestjs.com' }),

src/app.module.ts

@@ -1,8 +1,13 @@
-import { Injectable } from '@nestjs/common';
+import { Injectable } from '@nestjs/common'
+import { I18nService } from './i18n/i18n.service'
 
 @Injectable()
 export class AppService {
+  constructor(private readonly i18nService: I18nService) {}
+
   getHello(): string {
-    return 'Hello World!';
+    return this.i18nService.translate('ERRORS.USER_NOT_FOUND', {
+      firstName: 'John',
+    })
   }
 }

src/app.service.ts

@@ -0,0 +1,6 @@
+{
+  "HELLO": "Hello {firstName}",
+  "ERRORS": {
+    "USER_NOT_FOUND": "User {firstName} does not exist"
+  }
+}

src/assets/locales/en.json

@@ -0,0 +1,6 @@
+{
+  "HELLO": "Czesc {firstName}",
+  "ERRORS": {
+    "USER_NOT_FOUND": "Uzytkownik {firstName} nie istnieje"
+  }
+}

src/assets/locales/pl.json

@@ -0,0 +1,47 @@
+import {
+  ContextId,
+  ContextIdFactory,
+  ContextIdResolver,
+  ContextIdResolverFn,
+  ContextIdStrategy,
+  HostComponentInfo,
+} from '@nestjs/core'
+import { pick } from 'accept-language-parser'
+import { Request } from 'express'
+import { I18nService } from 'src/i18n/i18n.service'
+
+export class AggregateByLocaleContextIdStrategy implements ContextIdStrategy {
+  // A collection of context identifiers representing separate DI sub-trees per locale
+  private readonly locales = new Map<string, ContextId>()
+
+  attach(
+    contextId: ContextId,
+    request: Request,
+  ): ContextIdResolverFn | ContextIdResolver {
+    const localeCode =
+      pick(
+        I18nService.supportedLanguages,
+        request.headers['accept-language']!,
+      ) ?? I18nService.defaultLanguage
+
+    let localeSubTreeId: ContextId
+
+    if (this.locales.has(localeCode)) {
+      localeSubTreeId = this.locales.get(localeCode)!
+    } else {
+      // Construct a new context id
+      localeSubTreeId = ContextIdFactory.create()
+      this.locales.set(localeCode, localeSubTreeId)
+
+      // we can remove the locale context id after a certain period of time
+      // if not UserService instance is created once per locale
+      setTimeout(() => this.locales.delete(localeCode), 3000)
+    }
+
+    return {
+      payload: { localeCode },
+      resolve: (info: HostComponentInfo) =>
+        info.isTreeDurable ? localeSubTreeId : contextId,
+    }
+  }
+}

src/core/aggregate-by-locale.strategy.ts

@@ -0,0 +1,8 @@
+import { Module } from '@nestjs/common'
+import { I18nService } from './i18n.service'
+
+@Module({
+  providers: [I18nService],
+  exports: [I18nService],
+})
+export class I18nModule {}

src/i18n/i18n.module.ts

@@ -0,0 +1,18 @@
+import { Test, TestingModule } from '@nestjs/testing';
+import { I18nService } from './i18n.service';
+
+describe('I18nService', () => {
+  let service: I18nService;
+
+  beforeEach(async () => {
+    const module: TestingModule = await Test.createTestingModule({
+      providers: [I18nService],
+    }).compile();
+
+    service = module.get<I18nService>(I18nService);
+  });
+
+  it('should be defined', () => {
+    expect(service).toBeDefined();
+  });
+});

src/i18n/i18n.service.spec.ts

@@ -0,0 +1,45 @@
+import { Inject, Injectable, Scope } from '@nestjs/common'
+import type * as Schema from '../assets/locales/en.json'
+import * as en from '../assets/locales/en.json'
+import * as pl from '../assets/locales/pl.json'
+import { REQUEST } from '@nestjs/core'
+import format from 'string-format'
+
+type PathsToStringProps<T> = T extends string
+  ? []
+  : {
+      [K in Extract<keyof T, string>]: [K, ...PathsToStringProps<T[K]>]
+    }[Extract<keyof T, string>]
+
+type Join<T extends string[]> = T extends []
+  ? never
+  : T extends [infer F]
+    ? F
+    : T extends [infer F, ...infer R]
+      ? F extends string
+        ? `${F}.${Join<Extract<R, string[]>>}`
+        : never
+      : string
+
+@Injectable({ scope: Scope.REQUEST, durable: true })
+export class I18nService {
+  constructor(
+    @Inject(REQUEST) private readonly payload: { localeCode: string },
+  ) {}
+
+  public static readonly defaultLanguage = 'en'
+  public static readonly supportedLanguages = ['en', 'pl']
+  private readonly locales: Record<string, typeof Schema> = { en, pl }
+
+  translate(
+    key: Join<PathsToStringProps<typeof Schema>>,
+    ...args: Array<string | Record<string, unknown>>
+  ): string {
+    const locale =
+      this.locales[this.payload.localeCode ?? I18nService.defaultLanguage]
+
+    // To support dot notation: "ERRORS.USER_NOT_FOUND"
+    const text: string = key.split('.').reduce((o, i) => o[i], locale)
+    return format(text, ...args)
+  }
+}