Retrieve a function's type annotation that lists an object's properties along with their respective generic types

Question

Is there a way to determine and utilize an object's properties along with their generic types within the type annotation of a function?

I am looking for a method to deduce the type RequestData so that it can be correctly passed on to the request callback.

At present, I find myself employing type assertions when invoking this function. It would be beneficial if my IDE could automatically infer the type of data based on the Form object provided.

If you have any suggestions on how to avoid using any in my code example, kindly share them as well.

Thank you!

Example

class Field<T> {
  value: T;

  constructor(value: T) {
    this.value = value;
  }
}

class Form {
  // Collection of all Field objects within this Form
  fields: Record<string, Field<any>>;

  constructor({ fields }: { fields: Record<string, Field<any>>; }) {
    this.fields = fields;
  }
}



const submit = async <Response extends unknown, RequestData extends Record<string, unknown>>(
  form: Form,
  request: (data: RequestData) => Promise<Response>,
): Promise<Response> => {
  const fieldsAsGenerics = {} as RequestData; // Retrieve values stored in each form.field

  return await request(fieldsAsGenerics);
};



const myForm = new Form({
   fields: {
    name: new Field<string>('Charlie'),
    age: new Field<number | null>(66),
  }
});

submit(myForm, async (data) => {
  // The expected type for data is { [K in keyof typeof myForm.fields]: typeof myForm.fields[K] }
  // which, in this instance, translates to { name: string; age: number | null }
  // However, the actual type remains unknown
});

Playground Link

See also

  • TypeScript: how to extract the generic parameter from a type?
  • Pass Object Key As Generic

Answer №1

The core issue lies in the fact that your Form type lacks genericity, leading to a lack of information regarding the types of fields it contains. The compiler cannot differentiate between different instances of Form based on their field types, resulting in a uniform behavior for all Form objects.

To address this, we should make Form generic with respect to the type of Record it holds.

class Form<T extends Record<string, Field<any>>> {
  // Object containing all Field objects in this Form
  fields: T

  constructor({ fields }: { fields: T}) {
    this.fields = fields;
  }
}

Next, utilize a utility type to extract the RequestData type from the record type, essentially removing the Field wrapper.

type ExtractFieldValues<T extends Record<string, Field<any>>> = {
    [K in keyof T]: T[K] extends Field<infer V> ? V : never;
};

Adjust the submit function to align with the new types, requiring a type assertion during the unboxing process from Field instances. The compiler may not detect every property within the Fields type without this assertion:

const submit = async <Response extends unknown, Fields extends Record<string, Field<any>>>(
  form: Form<Fields>,
  request: (data: ExtractFieldValues<Fields>) => Promise<Response>,
): Promise<Response> => {
  const fieldsAsGenerics = {
      // Extract value stored in every form.field
  } as ExtractFieldValues<Fields>;
  // A type assertion will ALWAYS be required here:
  // the compiler can't infer that you're looping over every
  // property of the form type and extracting T from Field<T>.
  
  return await request(fieldsAsGenerics);
};

To avoid the need for a type assertion, consider storing values directly in the fields object without the Field wrapper, depending on your code's requirements and structure.

With these changes, usage scenarios can correctly infer types (confirm through mouseover in the provided playground link):

const myForm = new Form({
 fields: {
    name: new Field<string>(/*Field constructor argsargs*/),
    age: new Field<number | null>(/*Field constructor args*/)
  }
});

submit(myForm, async (data) => {
  // data should be of type { [K in keyof typeof myForm.fields]: typeof myForm.fields[K] }
  // which in this case is { name: string; age: number | null }
});

Playground link

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