Having trouble making generics work with extends in Typescript

I am facing an issue when trying to limit input to an object, but unfortunately, it is not working correctly:

displayModal<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.displayModal(component, options));
}

An error message indicates: 

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

I need some assistance in resolving this issue. What am I overlooking?

typescript

Answer №1

T represents any type that extends {[key: string]: any}. For instance, it could be defined as { requiredField: number }. When T is of this nature, the value {} cannot serve as a valid default for T due to its required fields. Therefore, the compiler prohibits assigning {} in scenarios where it would not be appropriate.

To navigate around this limitation, a common practice is to employ a type assertion:

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

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