Is \u006eew a valid Identifier?

Allen Wirfs-Brock allen at
Mon Nov 9 17:05:17 UTC 2015

> On Nov 9, 2015, at 6:55 AM, Andreas Rossberg <rossberg at> 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`.
> /Andreas


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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