const state = {
  prop1: 'value1',
  prop2: 'value2',
  prop3: 'value3',
}

type State = typeof state;

type Keys = (keyof State)

const makeSetProp = (path: Keys | undefined) => {
  // "setProp" is not null if "path" is not undefined
  const setProp = 
    path ?
      (value: string) => state[path] = value :
      null;
  // "data" is not null is "path" is not undefined
  const data =
    path ?
      'nextValue' :
      null;

  // "data" is always not null if "setProp" is not null, but the compiler does not recognize it
  if (setProp) setProp(data); // Argument of type 'string | null' is not assignable to parameter of type 'string'
  if (data) setProp(data); // Cannot invoke an object which is possibly 'null'.
  if (path) setProp(data); // Cannot invoke an object which is possibly 'null'. Argument of type 'string | null' is not assignable to parameter of type 'string'.
  if (setProp && data) setProp(data); // works
}