> On Jul 22, 2016, at 10:04 AM, Bradford Smith <[email protected]> > 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
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