Is \u006eew a valid Identifier?
allen at wirfs-brock.com
Mon Nov 9 17:05:17 UTC 2015
> On Nov 9, 2015, at 6:55 AM, Andreas Rossberg <rossberg at google.com> wrote:
> Allen, what was the motivation for allowing random escapes in
> identifiers but not in keywords? AFAICS, it would be simpler and more
> consistent to allow them anywhere and render "escape normalisation" a
> uniform prepass before tokenisation. IIUC, that's what other languages
> do. The current ES rules are far from ideal, and require jumping
> through extra hoops, in particular, to handle context-dependent
> keywords like `yield`.
Here are some references:
there are many others, and also there were earlier TC39 meeting discussions that I didn’t find in my quick search.
It’s a usability vs. implementor convience trade-off. the TC39 was to go with usability (and in particular readability).
(Also, my recollection is that in some TC39 discussions (that I didn’t find in my search) there were security concerns raised WRT allowing unicode escapes in keywords. Probably concerns about code injection filters not recognizing escaped keywords)
In ES6 (and I believe that Waldemar would claim in previous editions) unicode escapes cannot be handled with such a prepass. Essentially, escaped and non-escaped IdentifierName characters are only equated when doing identifier binding or property name lookups. It’s probably a misperception of the lexical grammar and static semantics that leads some implementors down the path of thinking that such a preps is reasonable.
Regarding `yield`, if it is written containing unicode escapes it is never a contextual keyword.
BTW, personally I I would be just fine with never allowing unicode escapes within IdentiferName. But that would be a web breaking change.
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