Get the right type in a function from multiple possible type in typescript

I'm having a bit of trouble figuring out how to solve my problem, despite the many answers I've already see on similar subjects.

I have the following types -- I'll add more later (one for each form roughly):

export interface FormError {
  formError: string
}

export interface InputState<T> {
  value: T
  onError: boolean
  validation: FormDataValidation[] | undefined
  errorMessage?: string
}

export interface WorkoutForm extends FormError {
  trainingType: InputState<trainingType>
  name: InputState<string>
  date: InputState<string>
}

export interface LoginForm extends FormError {
  email: InputState<string>
  password: InputState<string>
}

I have the following function where I'd like to be able to use a "generic" type so that I can use it with WorkoutForm or with LoginForm or with others that I'll create later.

I've tried to solve my problem with T but my linter gives me lots of errors every time.

public validate<T>(formData: T, index: Exclude<keyof T, 'formError'>): T {
    const validation = formData[index].validation; // << Property 'validation' does not exist on type 'T[Exclude ]'
    // ....... code
    return formData;
  }

I'd like to be able to use it like this, for example:

function handleFormData(index: Exclude<keyof LoginForm, 'formError'>, value: string): void {
  // ... some code
  const currentFormData = formData; // currentFormData type is LoginForm
  currentFormData[index].value = value;
  const validatedData = FormValidation.getInstance().validate<LoginForm>(currentFormData, index);
  // ... some code
}

Any tips or idea to solve this problem?

You should make validate() generic in both T, the type of formData, as well as K, the type of index. Additionally, you should constrain T and K so that K is a keylike type, and T is known to be an object with a property whose key type is K and whose value type is some InputState<any> (you can use unknown instead of any as long as InputState<T> is covariant in T, see https://stackoverflow.com/q/66410115/2887218 for more information). That can be written as Record<K, InputState<any>> using the Record<K, V> utility type. Like so:

function validate<
  T extends Record<K, InputState<any>>,
  K extends PropertyKey>(
    formData: T, index: K): T {
  const validation = formData[index].validation;
  return formData;
}

That compiles with no error, and now the rest of your code behaves as desired.

Playground link to code

<!-- edit -->