Why is this valid syntax?

Question

While refactoring some code I accidentally discovered that this is valid syntax (or at least, doesn't cause a parser error in Firefox):

const {} = somefunc();

somefunc returns an object and the curly's are supposed to contain variable names for destructuring, at the time I hadn't decided what those names would be so I temporarily left them empty.

The editor didn't highlight a syntax error, so out of curiosity I tested it, and was surprised that Firefox actually had no issue with this syntax.

Why is this syntax valid? Does it actually do something weird?

There is nothing weird about it, all it means is that nothing is being destructured from it — Shubham Khatri, May 06 '20 at 14:10

score 19 · Accepted Answer · edited Jun 20 '20 at 09:12

19

Yes, empty destructuring like that is perfectly fine, it's just useless. This is covered in Runtime Semantics: DestructuringAssignmentEvaluation

With parameter value.

ObjectAssignmentPattern: {}

Perform ? RequireObjectCoercible(value).

Return NormalCompletion(empty).

All it does (in RequireObjectCoercible) is require that the right side is not null nor undefined.

edited Jun 20 '20 at 09:12

Community

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answered May 06 '20 at 14:11

CertainPerformance

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"All it does (in RequireObjectCoercible) is require that the right side is not null nor undefined." ahh interesting, so it's not quite useless since it does that null check (although other methods are likely clearer and perhaps less code too, so this is probably not a sensible way to do only this) – DJL May 06 '20 at 14:19

score 13 · Answer 2 · edited Jul 19 '21 at 12:47

13

When destructuring an object you chose which properties you need:

const {a, c} = {'c': 1, 'b': 2, 'a': 3};

Thus having no parameters is acceptable, but useless nonetheless:

const {} = {};

edited Jul 19 '21 at 12:47

Brett Zamir

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answered May 06 '20 at 14:10

Mike Doe

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score 4 · Answer 3 · answered May 12 '20 at 19:46

It's just the limiting case of object destructuring.

While it may seem pointless to allow it, allowing edge cases like this is useful for automatically generated code, so the generator doesn't have to check that there are zero items and skip generating the code.

Why is this valid syntax?

3 Answers3