What causes "Cannot configure the test module when the test module has already been instantiated"?

Issue

here is my test :

describe('ValueService', () => {

  it('#getValue should return real value', () => {
    expect(true).toBeTruthy();
  });
});

And I have this error :

Failed: Cannot configure the test module when the test module has already been instantiated. Make sure you are not using inject before R3TestBed.configureTestingModule. Error: Cannot configure the test module when the test module has already been instantiated. Make sure you are not using inject before R3TestBed.configureTestingModule.

Solution

As discussed with the author, the problem arise when the TestBed is initialized outside a describe when having two spec files or more.

For example:

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      imports: [
        RouterTestingModule
      ],
      declarations: [
        AppComponent
      ],
    }).compileComponents();
  }));
describe('AppComponent', () => {
  it('should create the app', () => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.componentInstance;
    expect(app).toBeTruthy();
  });
 });

will instanciate a TestBed beforeEach tests, not only the spec file. Hence, if you have another .spec with a TestBed and a beforeEach it will be interpreted as 2 TestBed instanciated like this:

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      imports: [
        RouterTestingModule
      ],
      declarations: [
        AppComponent
      ],
    }).compileComponents();
  }));
  beforeEach(async(() => {
    TestBed.configureTestingModule({
      imports: [
        RouterTestingModule
      ],
      declarations: [
        AppComponent
      ],
    }).compileComponents();
  }));
describe('AppComponent', () => {
  it('should create the app', () => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.componentInstance;
    expect(app).toBeTruthy();
  });
 });

The error

Failed: Cannot configure the test module when the test module has already been instantiated.

will be right, since you instanciate two TestBed (but in two spec files).

To solve this problem, you must always put the TestBed definition (so the beforeEach) in a describe like this:

describe('AppComponent', () => {
  beforeEach(async(() => {
    TestBed.configureTestingModule({
      imports: [
        RouterTestingModule
      ],
      declarations: [
        AppComponent
      ],
    }).compileComponents();
  }));

  it('should create the app', () => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.componentInstance;
    expect(app).toBeTruthy();
  });
});

Answered By - JFPicard