Angular Unit Testing: Using Generics to Create Component

Question

I am trying to create a generic wrapper around TestBed.createComponent, which takes a type argument and creates the component for that type. However, the TestBed.createComponent function takes an argument of type Type<T>. I'm unable to create a Type<T> from the passed in generic T argument.

export function createTestHarness<T>(): TestHarness<T> {
  let component: T;
  let fixture: ComponentFixture<T>;

  fixture = TestBed.createComponent<T>(**PROBLEM AREA**);
  component = fixture.componentInstance;
  fixture.detectChanges();

  return new TestHarness<T>(component, fixture);
}

Is there a way to derive a Type<T> from the type passed in?

Daniel W Strimpel · Accepted Answer · 2019-05-09 16:16:58Z

5

One option you have is to use the Type<T> as a parameter to your function:

function createTestHarness<T>(type: Type<T>): TestHarness<T> {
  let component: T;
  let fixture: ComponentFixture<T>;

  fixture = TestBed.createComponent<T>(type);
  component = fixture.componentInstance;
  fixture.detectChanges();

  return new TestHarness<T>(component, fixture);
}

With the following usage:

const harness = createTestHarness(TestComponent);

Which will return a TestHarness<TestComponent>.

answered May 9, 2019 at 16:16

Daniel W Strimpel

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Comments

sonallux · Accepted Answer · 2019-05-09 16:12:20Z

0

Generics only exists at compile time and not at runtime. So you can not derive the type of T.

Get type of generic parameter

answered May 9, 2019 at 16:12

sonallux

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