Programmatic changes to Angular 2 component does not execute ngOnChanges in unit test

Question

I have a rather simple, working Angular 2 component. The component renders some div sibling elements based off of an array of values that I have sorted. This component implements OnChanges . The sorting functionality happens during the execution of ngOnChanges() . Initially, my @Input() attribute references an array of values that are unsorted. Once the change detection kicks in, the resulting DOM elements are sorted as expected.

I have written a Karma unit test to verify that the sorting logic in the component has taken place and that the expected DOM elements are rendered in sorted order. I programmatically set the component property in the unit test. After calling fixture.detectChanges() , the component renders its DOM. However, what I am ngOnChanges()` function, I see it is never executed. What is the correct way to unit test this behavior?

Here is my component:

@Component({
  selector: 'field-value-map-item',
  templateUrl: 'field-value-map-item.component.html',
  styleUrls: [ 'field-value-map-item.component.less' ]
})
export class FieldValueMapItemComponent implements OnChanges {

  @Input() data: FieldMapping
  private mappings: { [id: number]: Array<ValueMapping> }

  ngOnChanges(changes: any): void {
    if (changes.data) { // && !changes.data.isFirstChange()) {
      this.handleMappingDataChange(changes.data.currentValue)
    }
  }

  private handleMappingDataChange(mapping: FieldMapping) {

    // update mappings data structure
    this.mappings = {};
    if (mapping.inputFields) {
      mapping.inputFields.forEach((field: Field) => {
        field.allowedValues = sortBy(field.allowedValues, [ 'valueText' ])
      })
    }

    if (mapping.valueMap) {
      // console.log(mapping.valueMap.mappings.map((item) => item.input))
      // order mappings by outputValue.valueText so that all target
      // values are ordered.
      let orderedMappings = sortBy(mapping.valueMap.mappings, [ 'inputValue.valueText' ])

      orderedMappings.forEach((mapping) => {
        if (!this.mappings[mapping.outputValue.id]) {
          this.mappings[mapping.outputValue.id] = []
        }
        this.mappings[mapping.outputValue.id].push(mapping)
      })
    }
  }

}

The component template:

<div class="field-value-map-item">
  <div *ngIf="data && data.valueMap"
    class="field-value-map-item__container">
    <div class="field-value-map-item__source">
      <avs-header-panel
        *ngIf="data.valueMap['@type'] === 'static'"
        [title]="data.inputFields[0].name">
        <ol>
          <li class="field-value-mapitem__value"
            *ngFor="let value of data.inputFields[0].allowedValues">
            {{value.valueText}}
          </li>  
        </ol>

      </avs-header-panel>
    </div>
    <div class="field-value-map-item__target">
      <div *ngFor="let value of data.outputField.allowedValues">
        <avs-header-panel [title]="value.valueText">
          <div *ngIf="mappings && mappings[value.id]">
            <div class="field-value-mapitem__mapped-value"
              *ngFor="let mapping of mappings[value.id]">
              {{mapping.inputValue.valueText}}
            </div>
          </div>
        </avs-header-panel>
      </div>
    </div>
  </div>
</div>

Here is my unit test:

describe('component: field-mapping/FieldValueMapItemComponent', () => {
  let component: FieldValueMapItemComponent
  let fixture:   ComponentFixture<FieldValueMapItemComponent>
  let de:        DebugElement
  let el:        HTMLElement

  beforeEach(() => {

    TestBed.configureTestingModule({
      declarations: [
        FieldValueMapItemComponent
      ],
      imports: [
        CommonModule
      ],
      providers: [ ],
      schemas: [
        CUSTOM_ELEMENTS_SCHEMA
      ]
    })

    fixture = TestBed.createComponent(FieldValueMapItemComponent)
    component = fixture.componentInstance

    de = fixture.debugElement.query(By.css('div'))
    el = de.nativeElement
  })

  afterEach(() => {
    fixture.destroy()
  })

  describe('render DOM elements', () => {

    beforeEach(() => {
      spyOn(component, 'ngOnChanges').and.callThrough()
      component.data = CONFIGURATION.fieldMappings[0]
      fixture.detectChanges()
    })

    it('should call ngOnChanges', () => {
      expect(component.ngOnChanges).toHaveBeenCalled()   // this fails!
    })

  })


  describe('sort output data values', () => {

    beforeEach(() => {
      component.data = CONFIGURATION.fieldMappings[1]
      fixture.detectChanges()
    })

    it('should sort source field allowed values by their valueText property', () => {
      let valueEls = el.querySelectorAll('.field-value-mapitem__value')
      let valueText = map(valueEls, 'innerText')
      expect(valueText.join(',')).toBe('Active,Foo,Terminated,Zip')   // this fails
    })
  })

})

Answer 1

A solution to this problem is to introduce a simple Component in the unit test. This Component contains the component in question as a child. By setting input attributes on this child component, you will trigger the ngOnChanges() function to be called just as it would in the application.

Updated unit test (notice the simple @Component({...}) class ParentComponent at the top). In the applicable test cases, I instantiated a new test fixture and component (of type ParentComponent) and set the class attributes that are bound to the child components input attribute in order to trigger the ngOnChange() .

@Component({
  template: `
    <div id="parent">
      <field-value-map-item [data]="childData"></field-value-map-item>
    </div>
  `
})
class ParentComponent {
  childData = CONFIGURATION.fieldMappings[1]
}

describe('component: field-mapping/FieldValueMapItemComponent', () => {
  let component: FieldValueMapItemComponent
  let fixture:   ComponentFixture<FieldValueMapItemComponent>
  let de:        DebugElement
  let el:        HTMLElement

  beforeEach(() => {

    TestBed.configureTestingModule({
      declarations: [
        FieldValueMapItemComponent,
        ParentComponent
      ],
      imports: [
        CommonModule
      ],
      providers: [ ],
      schemas: [
        CUSTOM_ELEMENTS_SCHEMA
      ]
    })

    fixture = TestBed.createComponent(FieldValueMapItemComponent)
    component = fixture.componentInstance

    de = fixture.debugElement.query(By.css('div'))
    el = de.nativeElement
  })

  // ...

  describe('sort output data values', () => {

    let parentComponent: ParentComponent
    let parentFixture: ComponentFixture<ParentComponent>
    let parentDe: DebugElement
    let parentEl: HTMLElement

    beforeEach(() => {
      parentFixture = TestBed.createComponent(ParentComponent)
      parentComponent = parentFixture.componentInstance

      parentDe = parentFixture.debugElement.query(By.css('#parent'))
      parentEl = parentDe.nativeElement

      parentFixture.detectChanges()
    })

    it('should sort source field allowed values by their valueText property', () => {
      let valueEls = parentEl.querySelectorAll('.field-value-mapitem__value')
      let valueText = map(valueEls, 'innerText')
      expect(valueText.join(',')).toBe('Active,Foo,Terminated,Zip')
    })

    it('should sort target field mapped values by `inputValue.valueText`', () => {
      parentComponent.childData = CONFIGURATION.fieldMappings[2]
      parentFixture.detectChanges()

      let valueEls = parentEl.querySelectorAll('.field-value-mapitem__mapped-value')
      let valueText = map(valueEls, 'innerText')
      expect(valueText.join(',')).toBe('BRC,FreeBurrito,FreeTaco')
    })
  })

})

Answer 2

The other way is calling ngOnChanges directly.

it('should render `Hello World!`', () => {
  greeter.name = 'World';

  //directly call ngOnChanges
  greeter.ngOnChanges({
    name: new SimpleChange(null, greeter.name)
  });
  fixture.detectChanges();
  expect(element.querySelector('h1').innerText).toBe('Hello World!');
});

ref: https://medium.com/@christophkrautz/testing-ngonchanges-in-angular-components-bbb3b4650ee8