打字稿：无法让泛型与扩展一起使用

Question

I'm trying to restrict input to an object, but it fails:

showModal<T extends {[key: string]: any}, U>(component: Type<AbstractDialogComponent<T, U>>,
    options?: ModalDialogOptions & { context: T }): Observable<U> {

    options = { context: {}, viewContainerRef: this.vcRef,
        fullscreen: true , ...options || {} };
    return Observable.fromPromise(this.modal.showModal(component, options));
}

The error is: Type '{}' is not assignable to type T on the second line.

What am I missing here?

Answer 1

T can be any type that extends {[key: string]: any} , so it could for example be { requiredField: number } . If T is this type, the value {} is not a valid default for the T since T has required fields. Thus the compiler disallows the assignment of {} since there are calls where it would not be valid.

The usual way to get around this is to use a type assertion :

(...options || ({} as T))