mean-guide-frontend/src/app/error-interceptor.ts

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import {
  HttpHandler,
  HttpInterceptor,
  HttpRequest,
  HttpErrorResponse,
} from '@angular/common/http';
import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';
import { Injectable } from '@angular/core';
import { MatDialog } from '@angular/material/dialog';
import { ErrorComponent } from './error/error.component';

@Injectable()
export class ErrorInterceptor implements HttpInterceptor {

  constructor(private dialog: MatDialog){}

  intercept(req: HttpRequest<any>, next: HttpHandler) {
    return next.handle(req).pipe(
      catchError((error: HttpErrorResponse) => {
        const errorMessage = error.error.message ? error.error.message : 'An unknown error ocurred!'
        this.dialog.open(ErrorComponent, {
          data: {
            message:errorMessage
          }
        })
        return throwError(error)
      })
    );
  }
}