Microsoft/TypeScript
Ver no GitHubTreating `undefined` parameters as optional
Open
#12.400 aberto em 21 de nov. de 2016
CommittedHelp WantedSuggestion
Métricas do repositório
- Stars
- (48.455 stars)
- Métricas de merge de PR
- (Mesclagem média 6d 17h) (9 fundiu PRs em 30d)
Description
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? 🌷