Optional spread syntax

[justinfagnani]

Spread for arrays and function calls is not null-safe (though it is for object spread).

It'd be very convenient to write them as:

const arr = [...?listOne, ...?listTwo];
foo(...?args);

Instead of:

const arr = [...listOne || [], ...listTwo || []];
foo(...args || []);

[claudepache]

Indeed, but it is out of the scope of this proposal. I could add a note in the explainer for “possible future improvements”.

(It is neither a pressing issue, since || [] just works.)

[ljharb]

Also, with the nullish coalescing operator, I think foo(...args ?? []) and [...(listOne ?? [])] would precisely match the semantics you'd want ...? to have?

[justinfagnani]

It is neither a pressing issue, since || [] just works.

I agree it's not as pressing as the others, but the same just works bit could be said about them: a?.b is roughly (a || {}).b if you're expecting only an object or nullish value, or exactly (a ?? {}).b with nullish coalescing. But, if we thought this addition would slow anything down at all, I'd say definitely wait - the current 3 are far more important. Seems like the basic syntax is the big delay risk there currently though.

I think foo(...args ?? []) and [...(listOne ?? [])] would precisely match the semantics you'd want ...? to have

I would think in this case we could elide the empty array allocation completely.

[ljharb]

You could use an empty string as well, but i see what you mean.

[littledan]

I like @claudepache 's suggestion in https://github.com/tc39/proposal-optional-chaining/issues/55#issuecomment-372451816 . This proposal specifically tries to be relatively minimal in the constructs supported, compared to the CoffeeScript equivalent, to keep things easy to understand.

[claudepache]

Besides spread, there are other constructs that expect iterable and may be made optional:

• optional spread: [ ?...iterableOrNull ], f(?...iterableOrNull)
• optional loop: for (let x ?of iterableOrNull)
• optional delegation: ?yield* iterableOrNull

[claudepache]

Regret: for (let x of foo) and yield* foo should have been:

• for (let x = ...foo)
• yield ...foo

so that optional spread would have worked everywhere.

[js-choi]

Heh—for (let x of? foo) and yield *? foo, someday, perhaps?