ERROR in AppComponent cannot be used as an entry component

问题

Hey I'm new to using Angular and web development, I've been walking through a few tutorials.

When trying to incorporate use of my Firebase Database, upon compilation locally (using ng serve) my application is failing. Returning: "AppComponent cannot be used as an entry component"

If you could offer any help that'd be great. Thanks in advance :)

//app.component.ts
import { Component } from '@angular/core';
import {AuthService} from "./services/auth.service";
import {Router} from "@angular/router";
import { AngularFireDatabase } from 'angularfire2/database';
import { AngularFireList } from 'angularfire2/database';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})

class Event {
  constructor(public title) { }
}


export class AppComponent {
  title = 'qoqa';
  private eventCounter = 0;
  public events:AngularFireList<Event[]>;
  constructor(db: AngularFireDatabase, private authService: AuthService, private router: Router) {
    this.events= db.list('/events');
  }
  public AddEvent(): void {
    let newEvent = new Event(`My event #${this.eventCounter++}`);
    this.events.push([newEvent]);
  }
  signInWithFacebook() {
    this.authService.FacebookSignIn()
      .then((res) => {
        //this.router.navigate(['dashboard'])
      })
      .catch((err) => console.log(err));
  };
  signInWithGoogle() {
    this.authService.GoogleSignIn()
      .then((res) => {
        //this.router.navigate(['dashboard'])
      })
      .catch((err) => console.log(err));
  }
}

//app.module.ts
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { AngularFireModule } from '@angular/fire';
import { AngularFirestoreModule } from '@angular/fire/firestore';
import { AngularFireAuthModule } from '@angular/fire/auth';
import { environment } from '../environments/environment';
import { LoginComponent } from './login.component';
import { AppRoutingModule } from './app-routing';
import { AngularFireDatabaseModule } from 'angularfire2/database';
import { AuthService } from './services/auth.service';

import { AppComponent } from './app.component';
import { HomeComponent } from './home/home.component';
import { CreateEventComponent } from './create-event/create-event.component';
import { EventsComponent } from './events/events.component';
import { ProfileComponent } from './profile/profile.component';
import { SavedEventsComponent } from './saved-events/saved-events.component';
import { InvitationsComponent } from './invitations/invitations.component';
import { CreateQoqaComponent } from './create-qoqa/create-qoqa.component';



@NgModule({
  declarations: [
    AppComponent,
    LoginComponent,
    HomeComponent,
    CreateEventComponent,
    EventsComponent,
    ProfileComponent,
    SavedEventsComponent,
    InvitationsComponent,
    CreateQoqaComponent
  ],
  imports: [
    AngularFireModule.initializeApp(environment.firebase),
    AngularFirestoreModule,
    AngularFireAuthModule,
    AngularFireDatabaseModule,
    AppRoutingModule,
    BrowserModule
  ],
  providers: [AuthService],
  bootstrap: [AppComponent]
})
export class AppModule { }

回答1:

The @Component decoration should be used on the AppComponent not the Event.

Also check that AppComponent is declared on the AppModule.

回答2:

I just wanted to supply an example of the problem explained by @ibenjelloun.

@Component({ ... }), while looking a bit like a function, is a decorator, so it must precede the class that you want Angular to treat as a component. So make sure not to put classes between @Component({ ... }) and export class YourComponent.

So move Event to before/after AppComponent:

class Event {
    constructor(public title) { }
}

@Component({
    selector: 'app-root',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css']
})
export class AppComponent {
    ...

回答3:

This issue comes due to failed syntax or minor mistakes. I got the same issue while adding a new component to my app.component.ts file. while solving the issue I came to StackOverflow and thought to share my experience.

while checking all the files I found some syntax errors and minor mistakes in app.component.ts file.

After solving them and compiling I was able to get exact error messages.

One issue was having no proper file path mounting so I hope this might be the reason for the failure of your application at compile time.

import { AngularFireDatabase } from '**angularfire2/database**';
import { AngularFireList } from '**angularfire2/database**';

来源：https://stackoverflow.com/questions/52910879/error-in-appcomponent-cannot-be-used-as-an-entry-component