Angular 6 - can't resolve all parameters for AppComponent

Question

I am trying to build an application with Angular 6 and I am still setting everything up. But it seems there is something wrong with the dependency injection in my app.

Answer 1

I also experienced the same issue with Angular 8.2.13 + Typescript. Finally the trick was to use a @Inject('RefOnlyUsedForTesting'), even if the type is string.

export abstract class MyBaseClass {
  ....
  constructor(@Inject(ElementRef) elementRef: ElementRef,
          @Optional() @Inject('RefOnlyUsedForTesting') dep1: MyObject,
          @Optional() @Inject('RefOnlyUsedForTesting') dep2: string,
          @Optional() @Inject('RefOnlyUsedForTesting') dep3: string) {
  super();    
  ...
 }
}

Answer 2

I received the same error on Angular 8 app

Can't resolve all parameters for AppComponent: (?,?,?)

after changing 1 parameter type in the constructor().

I solved the problem by stop and restarting the app.

Answer 3

@Inject did the trick for me

import {Component, Inject} from '@angular/core';
import {TestService} from "./TestService";

@Component({
  selector: 'sh-home',
  styleUrls: ['./home.scss'],
  templateUrl: './home.html'
})
export class HomeComponent {

  constructor(@Inject(TestService) testService: TestService) {
    testService.sayHello();
  }
}

Answer 4

Make sure you have following import in polyfills:

import 'core-js/es7/reflect';

Answer 5

If you are using webpack and babel to build Angular, it is likely you are missing this babel plugin: babel-plugin-transform-typescript-metadata

Answer 6

I also got this issue but setting emitDecoratorMetadata": true in tsconfig solve the issue. Remember to restart the server.

tsconfig.spec.json

"compilerOptions": { "emitDecoratorMetadata": true, "outDir": "./out-tsc/spec",