Reason for strange ExponentiationExpression & UpdateExpression productions?

[Allen Wirfs-Brock]

> 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 <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 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.mozilla.org/pipermail/es-discuss/attachments/20160722/6889be27/attachment-0001.html>