Why do I get the NG0303 or NG0300 errors? (Angular Testing, Karma Jasmine) - ionic-framework

Question

I have several components (about,service,dashboard, etc) in which a header component is added. The application is running fine. However, I am getting errors when testing.
import { Component } from '#angular/core';
import { ComponentFixture, TestBed, waitForAsync } from '#angular/core/testing';
import { IonicModule } from '#ionic/angular';
import { SettingsPage } from './settings.page';
describe('SettingsPage', () => {
let component: SettingsPage;
let fixture: ComponentFixture<SettingsPage>;
beforeEach(waitForAsync(() => {
TestBed.configureTestingModule({
declarations: [ SettingsPage,MockHeaderComponent ],
imports: [IonicModule.forRoot()]
}).compileComponents();
fixture = TestBed.createComponent(SettingsPage);
component = fixture.componentInstance;
fixture.detectChanges();
}));
it('should create', () => {
expect(component).toBeTruthy();
});
});
#Component({
selector: 'app-header',
template: ''
})
class MockHeaderComponent {
}
First Error
Chrome 93.0.4577.82 (Windows 10): Executed 22 of 26 (1 FAILED) (0 secs / 5.88 secs)
ERROR: 'NG0303: Can't bind to 'headline' since it isn't a known property of 'app-header'.'
headline is a variable that I declared in my header component and use in my components to give me the correct title.
Of course, I already looked for solutions on the Internet and came across the following Stack Overflow post: Unit test Angular with Jasmine and Karma, Error:Can't bind to 'xxx' since it isn't a known property of 'xxxxxx'.
Therefore, I have imported and declared my HeaderComponent.
import { Component } from '#angular/core';
import { ComponentFixture, TestBed, waitForAsync } from '#angular/core/testing';
import { IonicModule } from '#ionic/angular';
import { HeaderComponent } from '../header/header.component';
import { SettingsPage } from './settings.page';
describe('SettingsPage', () => {
let component: SettingsPage;
let fixture: ComponentFixture<SettingsPage>;
beforeEach(waitForAsync(() => {
TestBed.configureTestingModule({
declarations: [ SettingsPage,HeaderComponent,MockHeaderComponent ],
imports: [IonicModule.forRoot()]
}).compileComponents();
fixture = TestBed.createComponent(SettingsPage);
component = fixture.componentInstance;
fixture.detectChanges();
}));
it('should create', () => {
expect(component).toBeTruthy();
});
});
#Component({
selector: 'app-header',
template: ''
})
class MockHeaderComponent {
}
Unfortunately, the following error occurred.
Chrome 93.0.4577.82 (Windows 10) SettingsPage should create FAILED
Failed: NG0300: Multiple components match node with tagname app-header. Find more at https://angular.io/errors/NG0300
error properties: Object({ code: '300' })
Error: NG0300: Multiple components match node with tagname app-header. Find more at https://angular.io/errors/NG0300
at throwMultipleComponentError (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:6779:1)
at findDirectiveDefMatches (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:10344:1)
at resolveDirectives (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:10158:1)
at elementStartFirstCreatePass (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:14772:1)
at ɵɵelementStart (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:14809:1)
at ɵɵelement (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:14888:1)
at SettingsPage_Template (ng:///SettingsPage.js:8:9)
at executeTemplate (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:9600:1)
at renderView (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:9404:1)
at renderComponent (node_modules/#angular/core/__ivy_ngcc__/fesm2015/core.js:10684:1)
Error: Expected undefined to be truthy.
at <Jasmine>
at UserContext.<anonymous> (src/app/components/settings/settings.page.spec.ts:24:23)
at ZoneDelegate.invoke (node_modules/zone.js/dist/zone-evergreen.js:364:1)
at ProxyZoneSpec.push.QpwO.ProxyZoneSpec.onInvoke (node_modules/zone.js/dist/zone-testing.js:292:1)
Chrome 93.0.4577.82 (Windows 10): Executed 26 of 26 (7 FAILED) (1.712 secs / 1.658 secs)
Can anyone tell me what I am doing wrong?
UPDATE 1:
My ComponentsModule class in which I integrate all reusable components:
import {NgModule} from '#angular/core';
import {HeaderComponent} from './header/header.component';
import { FooterComponent } from './footer/footer.component';
#NgModule({
declarations: [HeaderComponent, FooterComponent],
exports: [HeaderComponent, FooterComponent]
})
export class ComponentsModule{}
My html:
<ion-header>
<app-header [headline]="headlines.settings"></app-header>
</ion-header>
<ion-content>
</ion-content>
import { NgModule } from '#angular/core';
import { CommonModule } from '#angular/common';
import { FormsModule } from '#angular/forms';
import { IonicModule } from '#ionic/angular';
import { SettingsPageRoutingModule } from './settings-routing.module';
import { SettingsPage } from './settings.page';
import { ComponentsModule } from '../components.module';
#NgModule({
imports: [
CommonModule,
FormsModule,
IonicModule,
ComponentsModule,
SettingsPageRoutingModule
],
declarations: [SettingsPage]
})
export class SettingsPageModule {}
setting.page.ts
import { Component, OnInit } from '#angular/core';
import { lablesHeadlines } from 'src/environments/lables';
#Component({
selector: 'app-settings',
templateUrl: './settings.page.html',
styleUrls: ['./settings.page.scss'],
})
export class SettingsPage implements OnInit {
lablesHeadlines = lablesHeadlines;
headlines = lablesHeadlines;
constructor() { }
ngOnInit() {
}
}
setting.module.ts
import { NgModule } from '#angular/core';
import { CommonModule } from '#angular/common';
import { FormsModule } from '#angular/forms';
import { IonicModule } from '#ionic/angular';
import { SettingsPageRoutingModule } from './settings-routing.module';
import { SettingsPage } from './settings.page';
import { ComponentsModule } from '../components.module';
#NgModule({
imports: [
CommonModule,
FormsModule,
IonicModule,
ComponentsModule,
SettingsPageRoutingModule
],
declarations: [SettingsPage]
})
export class SettingsPageModule {}

Answer 1

Problem Solved. https://testing-angular.com/testing-components-with-children/#testing-components-with-children showed me the solution. That I write unit tests, I can do with
schemas: [NO_ERRORS_SCHEMA] to ignore the components in the top-level components. The low-level components are ready checked in their own tests.
I strongly advise to use the link above if you want to learn about testing in Angular.
Here is my Updated Sourcecode:
import { HttpClientTestingModule } from '#angular/common/http/testing';
import { ComponentFixture, TestBed, waitForAsync } from '#angular/core/testing';
import { IonicModule } from '#ionic/angular';
import { AboutClientServiceService } from 'src/app/services/about-client-service.service';
import { AppComponent } from 'src/app/app.component';
import { MockAboutClientService } from 'src/app/Mock/MockAboutClientService';
import { AboutPage } from './about.page';
import { Component } from '#angular/core';
import { NO_ERRORS_SCHEMA } from '#angular/core';
describe('AboutPage', () => {
let component: AboutPage;
let fixture: ComponentFixture<AboutPage>;
beforeEach(waitForAsync(() => {
TestBed.configureTestingModule({
declarations: [ AboutPage, MockHeaderComponent ],
schemas: [NO_ERRORS_SCHEMA],
imports: [IonicModule, HttpClientTestingModule],
providers: [AppComponent, AboutClientServiceService]
}).compileComponents();
TestBed.overrideComponent(
AboutPage,
{
set: {
providers: [
{ provide: AboutClientServiceService, useClass: MockAboutClientService }]
}
});
fixture = TestBed.createComponent(AboutPage);
component = fixture.componentInstance;
fixture.detectChanges();
}));
it('should create', (done) => {
expect(component).toBeTruthy();
expect(component.version).toBeDefined();
done();
});
});
#Component({
selector: 'app-header',
template: ''
})
class MockHeaderComponent {
}