Comment by ameliaquining
3 days ago
All of those languages except JavaScript have mutable strings; they just have different names and API shapes. Python has io.StringIO, Java has StringBuilder, Rust has String. (JavaScript has less need for this because JavaScript strings are ropes in most widely used implementations, reducing the overhead of concatenation, though the language spec doesn't require this.)
By your incredible criteria of things being mutable strings if they behave nothing like strings but can produce one, JavaScript absolutely does have a mutable string, it’s called Array. It’s also a mutable integer.
I don't consider JavaScript's Array to count because using it as a mutable string incurs a significant amount of additional overhead, because it has to box every character and check at runtime the type of each element. This is not true of the APIs that I listed.