debugElement return null in my angular5 test - unit-testing

Question

I wish to test an anguar5 component using a host component as described in angular.io doc.
But my test keep failing because of
TypeError: Cannot read property 'query' of null
at UserContext.<anonymous> (http://localhost:9876/base/config-webpack/spec-bundle.js:293832:39)
at ZoneDelegate../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (http://localhost:9876/base/config-webpack/spec-bundle.js:288418:26)
at ProxyZoneSpec../node_modules/zone.js/dist/proxy.js.ProxyZoneSpec.onInvoke (http://localhost:9876/base/config-webpack/spec-bundle.js:287920:39)
at ZoneDelegate../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (http://localhost:9876/base/config-webpack/spec-bundle.js:288417:32)
at Zone../node_modules/zone.js/dist/zone.js.Zone.run (http://localhost:9876/base/config-webpack/spec-bundle.js:288168:43)
at UserContext.<anonymous> (http://localhost:9876/base/config-webpack/spec-bundle.js:287799:34)
at attempt (http://localhost:9876/base/node_modules/jasmine-core/lib/jasmine-core/jasmine.js:4289:46)
at ZoneQueueRunner.QueueRunner.run (http://localhost:9876/base/node_modules/jasmine-core/lib/jasmine-core/jasmine.js:4217:20)
at ZoneQueueRunner.QueueRunner.execute (http://localhost:9876/base/node_modules/jasmine-core/lib/jasmine-core/jasmine.js:4199:10)
at ZoneQueueRunner../node_modules/zone.js/dist/jasmine-patch.js.jasmine.QueueRunner.ZoneQueueRunner.execute (http://localhost:9876/base/config-webpack/spec-bundle.js:287827:42)
Indeed, when I log my fixture.debugElement, it return null.
my test code is :
import {} from 'jasmine';
import { Component } from '#angular/core';
import { TestBed, ComponentFixture } from '#angular/core/testing';
import { By } from '#angular/platform-browser';
import { DropDownListComponent } from './dropDownList.component';
#Component({
template: '<dropdown-list [valuesList]="valuesList" [selectedValue]="valuesSelected" ></dropdown-list>'
})
class TestComponent {
valuesList = [{label: 'test_select', value:'test'}, {label: 'test_select2', value:'test2'}];
valuesSelected = {label: 'test_select', value:'test'};
}
describe('DropDownListComponent', () => {
let fixture, component;
beforeEach(() => {
TestBed.configureTestingModule({
declarations: [TestComponent, DropDownListComponent]
}).compileComponents();
});
it('should display selectedValue', () => {
fixture = TestBed.createComponent(TestComponent);
component = fixture.componentInstance;
fixture.detectChanges();
console.log(fixture.isStable());
console.log(fixture);
console.log(component);
console.log(fixture.debugElement);
//const fixture = TestBed.createComponent(TestComponent);
let de = fixture.debugElement.query(By.css(".value-container"));
let el = de.nativeElement;
expect(el.textContent).toContain('test_select');
});
});

Answer 1

When you write a test you need to test only your component / service / pipe / directive etc, but not its dependencies.
From the code above I assume DropDownListComponent has a DI dependency that wasn't declared in providers of TestBed and it causes the issue. Anyway in this context it should be a mock, not a real component.
If you want to test DropDownListComponent - then please share its code. Without understanding its interface it's hard to guess how to write tests for it.
You can use ng-mocks to mock it and then you would only need to test that [valuesList] and [selectedValue] got right values.
Also to compile all components correctly you need to handle compileComponents's promise.
describe('TestComponent', () => {
beforeEach(() => {
return TestBed.configureTestingModule({
declarations: [TestComponent, MockComponent(DropDownListComponent)],
}).compileComponents();
});
it('should display selectedValue', () => {
const fixture = MockRender(TestComponent);
const dropdown = MockHelper.findOrFail(fixture.debugElement, DropDownListComponent);
expect(dropdown.valuesList).toEqual(fixture.point.valuesList);
expect(dropdown.selectedValue).toEqual(fixture.point.valuesSelected);
});
});
Profit.