Experiment for dynamic interfaces

DefinedWhen.ts

/**
 * Construct a type based off T where TDefinedField is defined when
 * T matches TDefinedState.
 */
type DefinedWhen<
  T extends Record<string, any>,
  TDefinedField extends keyof T,
  TDefinedState extends Partial<T>,
> =
  | (Omit<T, TDefinedField | keyof TDefinedState> &
      TDefinedState &
      Record<TDefinedField, Exclude<T[TDefinedField], undefined>>)
  | {
      [DefinedStateKey in keyof TDefinedState]: {
        [K in keyof T]: K extends DefinedStateKey
          ? Exclude<T[K], TDefinedState[K]>
          : T[K];
      };
    }[keyof TDefinedState];

// =======================================================================================
// Example:
// ---------------------------------------------------------------------------------------

type Payload = DefinedWhen<
  {
    isFetched: boolean;
    hasError: boolean;
    data: string | undefined;
    dataType: string | undefined;
    meta: string | undefined;
  },
  "data" | "dataType" | "meta",
  { isFetched: true; hasError: false }
>;

function handleResponse({
  isFetched,
  hasError,
  data,
  dataType,
  meta,
}: Payload) {
  data; // string | undefined
  // ^?
  dataType; // string | undefined
  // ^?
  meta; // string | undefined
  // ^?

  if (isFetched === true && hasError === false) {
    data; // string
    // ^?
    dataType; // string
    // ^?
    meta; // string
    // ^?
  }
}