Shallow HOC with Enzyme and TypeScript

Question

I have a HOC to test, during shallow mount I should to call some class methods:

it('Should not call dispatch', () => {
        const dispatch = jest.fn()
        const WrappedComponent = someHoc(DummyComponent)
        const instance = shallow(
          <WrappedComponent
            dispatch={dispatch}
          />,
        ).instance() as WrappedComponent
        instance.someMethod()
        expect(dispatch).toHaveBeenCalledTimes(0)
})

test works fine but TS compiler throws an error

 Cannot find name 'WrappedComponent'.

And it is right because WrappedComponent is not a type or class, but if I remove the

 as WrappedComponent

line, TS throws an error

Property 'someMethod' does not exist on type 'Component<{}, {}, any>'.

Also, it does not compile if I change that line as

as typeof WrappedComponent

someHoc description:

import ...

interface State {
  /*state*/
}

interface Props {
  dispatch: Dispatch<Action>
  /*props*/
}

export someHoc = <T extends {}>(
  ChildComponent: React.ComponentClass<T>,
) => {
  class Wrapper extends React.PureComponent<T & Props, State> {

    someMethod = () => {
       /*do smth*/
    }

    render() {
      return (
        <div>
          <ChildComponent {...this.props} />
        </div>
      )
    }
  }

  return Wrapper
}

How can I type the HOC instance? Thanks.

Esmond

What's someHoc? How is it typed? Also, it does not compile if I change that line as - what is the error? – Such

@estus just added some description – Esmond

Answer 1

It is expected that functions that have variable return value types that can be parametrized are generics. shallow is a generic:

export function shallow<C extends Component, P = C['props'], S = C['state']>(node: ReactElement<P>, options?: ShallowRendererProps): ShallowWrapper<P, S, C>;
export function shallow<P>(node: ReactElement<P>, options?: ShallowRendererProps): ShallowWrapper<P, any>;
export function shallow<P, S>(node: ReactElement<P>, options?: ShallowRendererProps): ShallowWrapper<P, S>;

It likely should be used as:

const instance = shallow<typeof WrappedComponent>(
  <WrappedComponent
    dispatch={dispatch}
  />,
).instance();

There currently seem to be problems in Enzyme typings with using generic parameters to infer component type in ShallowWrapper.

A workaround that secures type safety down the test is to assert the type:

const instance = shallow(
  <WrappedComponent
    dispatch={dispatch}
  />,
)
.instance() as any as InstanceType<typeof WrappedComponent>;

Such

the same error ` Cannot find name 'WrappedComponent'.` And Property 'someMethod' does not exist on type 'Component<{}, {}, any>'. if I use typeof WrappedComponent – Esmond

Works if do shallow<typeof WrappedComponent> and .instance() as any, thanks! – Esmond

Yes, that was a typo, it's typeof. On second thought, this looks like a bug in Enzyme typings. I was just about to suggest to type it as .instance() as any as typeof WrappedComponent and move along. – Such

.instance() as any as typeof WrappedComponent is not working, so yes, looks like as some kind of bug. – Esmond

Sorry, sure, it's InstanceType<typeof WrappedComponent>, since typeof WrappedComponent is the constructor itself. Updated for clarity. – Such

Answer 2

This is not about HOC, but in case anyone finds this after Googling for hours, try updating @types/react to the latest version (I went from 18.0.14 to 18.2.38).

Which allowed me to use (unmodified)

const about = shallow(<About />);

given

export default function About() {
  const foo = true;
  return (
    <div>Blabla</div>
  )
}

with no problems.

Iamb