Coolwust wrote:
I have one more question, why `yield` and `in` keywords are so special? When can I treat `yield` and `in` as the non-reserved-word?
You can't treat `in` as an unreserved identifier, in any event. It has been reserved since JS1 in 1995, de-jure in ECMA-262 Edition 1 (ES1).
These are different, their histories differ. ES6 (after ES4 but without opt-in versioning) supports `yield` in generator functions but not elsewhere, because extant code on the Web over the last almost-20-years uses `yield` as an identifier in plain functions and/or global code.
`in` needs special treatment due to the ES3 grammatical ambiguity that would otherwise allow two ways of parsing the left sentential form `for (var x = y in z ...` (where the ... is meta). ECMA-262 aspires to specify an LR(1) grammar with lookahead restrictions and error correction procedures such as ASI. Allowing `in` expressions in variable intiialisers at the front of `for` loops would make the grammar not LR(k) for any k. (The potential ambiguity arose first in ES3 because that was when the `in` operator was added.)
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