Reason for strange ExponentiationExpression & UpdateExpression productions?

[Bradford Smith]

Thanks!

On Fri, Jul 22, 2016 at 11:14 AM Allen Wirfs-Brock <allen at wirfs-brock.com>
wrote:

>
> On Jul 22, 2016, at 10:04 AM, Bradford Smith <bradfordcsmith at google.com>
> wrote:
>
> A brief search didn't turn up any test cases that would enlighten me
> regarding why the rules for UpdateExpression are:
>
> UpdateExpression :
>     { ++ | -- } UnaryExpression
>     LeftHandSideExpression { ++ | -- }
>
> You can't increment or decrement anything that isn't a
> LeftHandSideExpression, so why does the syntax rule allow UnaryExpression
> as an argument for update in the prefix case?
>
>
> It’s basically preserve legal syntax that extends all the way back to the
> ES1 specification:
> http://ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf
>
>
> that syntax means that something like:
>
> if (false) ++ -42;
>
> Is syntactically valid.  A program containing such a line will load and
> execute without error.
>
> Changing the grammar (or static semantics rules) to make that illegal
> would be a ”breaking change” that might cause some valid existing scripts
> to suddenly fail to load.
>
> Allen
>
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