Defining dynamic modules
So far in this series, we’ve defined many different modules that can group provides and controllers. None of them was customizable, though. In this article, we…
So far in this series, we’ve defined many different modules that can group provides and controllers. None of them was customizable, though. In this article, we learn how to create dynamic modules that are customizable, making them easier to reuse. While doing that, we rewrite some of our old code to use dynamic modules. If you want to know more about modules in general, check out API with NestJS #6. Looking into dependency injection and modules To better illustrate why we might want to use dynamic modules, let’s look at the EmailModule we’ve defined in the API with NestJS #25. Sending scheduled emails with cron and Nodemailer article. email.module.ts import { Module } from '@nestjs/common'; import EmailService from './email.service'; import { ConfigModule } from '@nestjs/config'; @Module({ imports: [ConfigModule], controllers: [], providers: [EmailService], exports: [EmailService], }) export class EmailModule {}The above module contains the EmailService provider and exports it. Let’s look under the hood: email.service.ts import { Injectable } from '@nestjs/common'; import { createTransport } from 'nodemailer'; import * as Mail from 'nodemailer/lib/mailer'; import { ConfigService } from '@nestjs/config'; @Injectable() export default class EmailService { private nodemailerTransport: Mail; constructor(private readonly configService: ConfigService) { this.nodemailerTransport = createTransport({ service: configService.get('EMAIL_SERVICE'), auth: { user: configService.get('EMAIL_USER'), pass: configService.get('EMAIL_PASSWORD'), }, }); } sendMail(options: Mail.Options) { return this.nodemailerTransport.sendMail(options); } }The crucial thing to notice above is that our EmailService will always have the same configuration. So, for example, we can’t have a separate email for password confirmation and a separate email for the newsletter. To deal with this issue, we can create a dynamic module. Creating a dynamic module To create a dynamic module, let’s add a static method to the EmailModule. email.module.ts import { DynamicModule, Module } from '@nestjs/common'; import EmailService from './email.service'; import { EMAIL_CONFIG_OPTIONS } from './constants'; import EmailOptions from './emailOptions.interface'; @Module({}) export class EmailModule { static register(options: EmailOptions): DynamicModule { return { module: EmailModule, providers: [ { provide: EMAIL_CONFIG_OPTIONS, useValue: options, }, EmailService, ], exports: [EmailService], }; } }Above, instead of putting our module definition inside the @Module({}) decorator, we return it from the register method. We allow the user to provide the EmailOptions as the argument to the register function. emailOptions.interface.ts interface EmailOptions { service: string; user: string; password: string; } export default EmailOptions;We also define a unique EMAIL_CONFIG_OPTIONS constant in a separate file. constants.ts export const EMAIL_CONFIG_OPTIONS = 'EMAIL_CONFIG_OPTIONS'; We can also use a Symbol. In the register() method, we add the EMAIL_CONFIG_OPTIONS to the list of providers along with the option’s value. Thanks to the above, the configuration will be available in the EmailService through the @Inject() decorator. email.service.ts import { Inject, Injectable } from '@nestjs/common'; import { createTransport } from 'nodemailer'; import { EMAIL_CONFIG_OPTIONS } from './constants'; import Mail from 'nodemailer/lib/mailer'; import EmailOptions from './emailOptions.interface'; @Injectable() export default class EmailService { private nodemailerTransport: Mail; constructor(@Inject(EMAIL_CONFIG_OPTIONS) private options: EmailOptions) { this.nodemailerTransport = createTransport({ service: options.service, auth: { user: options.user, pass: options.password, }, }); } sendMail(options: Mail.Options) { return this.nodemailerTransport.sendMail(options); } }Thanks to our approach, we can now configure the module when importing it. emailConfirmation.module.ts import { Module } from '@nestjs/common'; import { EmailConfirmationService } from './emailConfirmation.service'; import { ConfigModule } from '@nestjs/config'; import { EmailModule } from '../email/email.module'; import { JwtModule } from '@nestjs/jwt'; import { EmailConfirmationController } from './emailConfirmation.controller'; import { UsersModule } from '../users/users.module'; @Module({ imports: [ ConfigModule, EmailModule.register({ service: 'gmail', user: 'email.account@gmail.com', password: 'mystrongpassword', }), JwtModule.register({}), UsersModule, ], providers: [EmailConfirmationService], exports: [EmailConfirmationService], controllers: [EmailConfirmationController], }) export class EmailConfirmationModule {} Asynchronous configuration There is a significant drawback to the above approach because we no longer can use the ConfigService when setting up our email provider. Fortunately, we can deal with this issue by creating an asynchronous version of our register method. It will have access to the dependency injection mechanism built into NestJS. Our registerAsync method will also receive the options, but the type used for its argument is slightly different than register.EmailModule.registerAsync({ imports: [ConfigModule], inject: [ConfigService], useFactory: (configService: ConfigService) => ({ service: configService.get('EMAIL_SERVICE'), user: configService.get('EMAIL_USER'), password: configService.get('EMAIL_PASSWORD'), }), }),Let’s go through all of the properties above: imports – a list of modules we want the EmailModule to import because we need them in useFactory, inject – a list of providers we want NestJS to inject into the context of the useFactory function, useFactory – a function that returns the value for our EMAIL_CONFIG_OPTIONS provider. A straightforward way to create a type with the above properties is to use the ModuleMetadata and FactoryProvider interfaces. emailOptions.type.ts import { ModuleMetadata } from '@nestjs/common'; import EmailOptions from './emailOptions.interface'; import { FactoryProvider } from '@nestjs/common/interfaces/modules/provider.interface'; type EmailAsyncOptions = Pick<ModuleMetadata, 'imports'> & Pick<FactoryProvider