An Angular Universal interceptor which handles HTTP requests to relative routes to local files. Fixes prerender errors for Angular applications which do this.

universal-interceptor.ts

import {Injectable, Inject, Optional} from '@angular/core';
import {
  HttpInterceptor, 
  HttpHandler, 
  HttpRequest, 
  HttpHeaders, 
  HttpResponse, 
  HttpErrorResponse
} from '@angular/common/http';
import {Request} from 'express';
import {REQUEST} from '@nguniversal/express-engine/tokens';
import {of, throwError} from "rxjs";
import {catchError} from "rxjs/operators";
import * as fs from "fs";
import * as path from "path";
import * as process from "process";

@Injectable()
export class UniversalInterceptor implements HttpInterceptor {

  constructor(@Optional() @Inject(REQUEST) protected request: Request) {}

  intercept(req: HttpRequest<any>, next: HttpHandler) {
    let serverReq: HttpRequest<any> = req;
    if (this.request) {
      let newUrl = `${this.request.protocol}://${this.request.get('host')}`;
      if (!req.url.startsWith('/')) {
        newUrl += '/';
      }
      newUrl += req.url;
      serverReq = req.clone({url: newUrl});
    }
    return next.handle(serverReq).pipe(
      catchError((error: HttpErrorResponse) => {
        console.error(`HTTP Request for ${req.url} failed, trying direct filesystem access.`);
        try{
          const filePath = path.join(process.cwd(), 'src', req.url);
          let encoding = 'utf8';
          if(req.responseType == 'arraybuffer'){
            encoding = undefined;
          }
          let body = fs.readFileSync(filePath, encoding);

          if(req.responseType == 'json'){
            body = JSON.parse(body);
          }

          return of(new HttpResponse(
            { status: 200, body: body }
          ));
        }catch(e){
          return throwError(e);
        }
      })
    );
  }
}