What are the best practices for utilizing generics effectively?

A series of interfaces has been defined:

export interface FormData<T extends ControlData = any> {
  [type: string]: T;
}

export type FormResult<T extends FormData> = {
  [type in keyof T]: T[type];
};

export interface ControlData<T = any> {
  value: T;
}

export interface ButtonSelectControlData<T> extends ControlData<T> {
  query: string;
}

export interface RoutesAddCityData extends FormData {
  cityId: ButtonSelectControlData<number>;
  routeId: ControlData<number>;
}

When utilizing the FormResult function:

(data: FormResult<RoutesAddCityData>) => {
  // ...
}

Upon inspection, it is noted that data.cityId is of type

ButtonSelectControlData<number>
and data.routeId is of type ControlData<number>. However, I am interested in seeing the type number in both cases. Is there a way to achieve this?

Answer №1

It seems like you're looking to have the FormResult type extract the underlying type parameters from both ButtonSelectControlData and ControlData. One approach to achieve this is by using a technique called a conditional type:

type FormResult<T> = {
  [K in keyof T]: T[K] extends ControlData<infer U>
    ? U
    : never         
}

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