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javascriptdestructuring

Why is this valid syntax?

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?

Solution

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

    With parameter value.

    ObjectAssignmentPattern: {}

    1. Perform ? RequireObjectCoercible(value).

    2. Return NormalCompletion(empty).

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