Why checkbox checked property doesn't change after detectChanges()

Question

I try to change checkbox checked property by changing model but it doesn't work. I cannot understand this behavior.

Template:

<input type="checkbox" [(ngModel)]="model"/>

Testing Code:

    it('should be checked after click on unchecked checkbox', () => {
        const checkbox: HTMLInputElement = fixture.nativeElement.querySelector('input');

        component.model = true;
        fixture.detectChanges();
        expect(checkbox.checked).toBeTruthy();
    });

Full code: https://stackblitz.com/edit/angular-testing-checkbox?file=app/app.component.spec.ts

Answer 1

Because angular change detection is asynchronous. Your expect() statement is executed before detectChanges(). Use angular testing async() function for this case:

   it('should be checked after click on unchecked checkbox', async( () => {
        const checkbox: HTMLInputElement = fixture.nativeElement.querySelector('input');    
        component.model = true;
        fixture.detectChanges();    
        fixture.whenStable().then(()=> {
          expect(checkbox.checked).toBeTruthy();
        })   
    }));