Wes Garland wrote:
If four-byte escapes are statically rejected in BRS-on, we have a problem -- we should be able to use old code that runs in either mode unchanged when said code only uses characters in the BMP.
We've been over this and I conceded to Allen that "four-byte escapes" (I'll use \uXXXX to be clear from now on) must work as today with BRS-on. Otherwise we make it hard to impossible to migrate code that knows what it is doing with 16-bit code units that round-trip properly.
Accepting both 4 and 6 byte escapes is a problem, though -- what is "\u123456".length? 1 or 3?
This is not a problem. We want .length to distribute across concatenation, so 3 is the only answer and in particular ("\u1234" + "\u5678").length === 2 irrespective of BRS.
If we accept "\u1234" in BRS-on as a string with length 5 -- as we do today in ES5 with "\u123".length===4 -- we give developers a way to feature-test and conditionally execute code, allowing libraries to run with BRS-on and BRS-off.
Feature-testing should be done using a more explicit test. API TBD, but I don't think breaking "\uXXXX" with BRS on is a good idea.
I agree with you that Roozbeh is hardly used, so it can take the hit of having to feature-test the BRS. The much more common case today is JS code that blithely ignores non-BMP characters that make it into strings as pairs, treating them blindly as two "characters" (ugh; must purge that "c-word" abusage from the spec).
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