NestJS module to serve a single page app and set its base href from an environment variable on startup

serve-static-files.module.ts

import { join } from 'path';

import { Module } from '@nestjs/common';
import { ServeStaticModule } from '@nestjs/serve-static';
import { replaceInFile } from 'replace-in-file';

const STATIC_FILES_DIR = join(__dirname, 'client');
const YOUR_GLOBAL_PREFIX = 'api'; // Typically imported from some other file

@Module({
  imports: [
    // For deployments, serve frontend's dist files from "client" folder on disk
    // relative to main.js; see https://github.com/nestjs/serve-static
    ServeStaticModule.forRoot({
      rootPath: STATIC_FILES_DIR,
      // Serve static files only from non-api paths so that 404 is returned
      // instead of index.html when an API URL doesn't match an API endpoint:
      renderPath: new RegExp(`^/(?!${YOUR_GLOBAL_PREFIX}/).*`),
    }),
    ApiUtilConfigModule,
  ],
  exports: [ServeStaticModule],
})
export class ServeStaticFilesModule {
  constructor(private apiConfigSvc: ApiConfigService) {
    this.writeBaseHref();
  }

  /**
   * Set the frontend's base href tag with the value from the environment
   * variable by writing it to the index.html file when the server starts.
   *
   * This allows the same static files to be deployed at any URL e.g. by
   * a load balancer.
   */
  private async writeBaseHref() {
    const baseHref = process.env.BASE_HREF || '/';
    const indexHtmlPath = join(STATIC_FILES_DIR, 'index.html');
    const newBaseHrefTag = `<base href="${baseHref}"`;
    try {
      const result = await replaceInFile({
        files: indexHtmlPath,
        from: /<base\s+href=".*"/,
        to: newBaseHrefTag,
      });
      console.log(
        `Wrote base href "${baseHref}" to disk at "${indexHtmlPath}". Result:`,
        result,
      );
    } catch (error) {
      const details = error instanceof Error ? error.message : error;
      console.error(
        'Error writing base href. Base href may not have been set. Details:',
        details,
      );
    }
  }
}