Treating `undefined` parameters as optional

tsc: 2.2.0-dev.20161116

Discover this behavior from https://github.com/acdlite/flux-standard-action/pull/45/commits/78a9065914b2ca4848dfba8fc0b47c54e2d0e319

This is the original code:

export interface FSA<Payload, Meta> {
  ...
  payload?: Payload;
  ...
}

However, this does not work well with type guard:

function isSomeAction(action: any): action is FSA<{ message: string }, void> {
  return true;
}

let action = {...};
if (isSomeAction(action)) {
  // `action.payload` may be undefined.
  console.log(action.payload.message);
}

Since generic type can be anything, including undefined, I'm considering to remove the optional designation:

export interface FSA<Payload, Meta> {
  ...
  payload: Payload;
  ...
}

// now type guard works
let action = {...};
if (isSomeAction(action)) {
  console.log(action.payload.message);
}

However, now the creation code fail:

function createSomeAction(): FSA<undefined, undefined> {
  // error: `payload` and `meta` are required
  return { type: 'SOME_ACTION' };
}

The workaround is to depends on infer type:

function createSomeAction() {
  return { type: 'SOME_ACTION' };
}

or specify it:

function createSomeAction(): FSA<undefined, undefined> {
  return { 
    type: 'SOME_ACTION',
    payload: undefined,
    meta: undefined
  };
}

Is there a better solution? 🌷

Inizia con gli esempi dell’interfaccia FSA<Payload, Meta> e gli snippet type-guard e createSomeAction nell’issue. Confronta il comportamento quando payload e meta sono opzionali con quello quando sono obbligatori, e determina quale risultato dovrebbe preservare sia il narrowing sia la creazione di azioni con valori undefined.