JavaScript has mixed UCS2 and UTF16 api. Some methods like codePointAt, fromCodePoint, toUpperCase/toLowerCase, for..of str, Array.from(str), normalize, regex's test are unicode aware. The rest are USC2 include substr and length. But it doesn't really make much difference...
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The strings are inherently not UTF-16 because they can contain invalid Unicode. UTF-16 doesn't support any string that isn't also supported by UTF-8. There is no lossy conversion between UTF-8 and UTF-16. They're different encodings of the same thing able to represent it all.
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C++ strings also can contain and produce invalid UTF8. Does it mean C++ not support UTF8? =)
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std::string is just a dynamic array of bytes able to manage a terminating NUL for C string compatibility. It doesn't have any specific encoding. Being able to stick UTF-8 in std::string doesn't mean it provides any form of UTF-8 support or that it's a UTF-8 string type.
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You still haven't answered, is it possible to conclude that C++ does not support UTF8? Yes or no?
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C strings and std::string are not Unicode string types and while they can hold UTF-8 they are not UTF-8 string types and do not provide any actual UTF-8 support. They're arrays of bytes. C++ std::string is implemented as a library and other libraries can implement string types.
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There's no point of making this disingenuous strawman argument. The fact is that JavaScript has legacy, broken strings and hasn't been fixed to support what Unicode has been for the past 21 years. JavaScript can be easily fixed rather than propagating brokenness further.
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There are people born after UCS2 was obsoleted who are now 20. Having 21 years to migrate to having real UTF-16 is a long time.
WebAssembly is not inherently tied to JavaScript and shouldn't be harmed to accommodate JavaScript not moving on from the 90s. Fix JavaScript instead.
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Let us then declare UTF8 obsolete as well. After all, it does not support grapheme clusters. Rob Pike and Kenneth Thompson developed UTF8 for Plan 9 back in 1992, which means he is almost 30 years old.
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It's significantly older than JavaScript and yet it properly supports modern Unicode while also being the native format everyone uses. It supports grapheme clusters just fine. Valid UTF-8 is valid Unicode just as valid UTF-16 is valid Unicode. JS strings are not UTF-16 strings.