app.middleware.ts

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response } from 'express';
import { join } from 'path';

@Injectable()
export class ServeHTMLMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: () => void) {
    // here you can check if the requested path is your api endpoint, if that's the case then we have to return next()
    if (req.path.includes('graphql')) {
      return next();
    }

    // change the path to the correct html page path in your project
    res.sendFile(join(process.cwd(), '../client/dist/index.html'));
  }
}