V8 apparently stores all-ASCII strings as ASCII, so stuff like HTML tag names or base64 blobs doesn’t double in size. Python does this too. In Python 2, and i

V8 apparently stores all-ASCII strings as ASCII, so stuff like HTML tag names or base64 blobs doesn’t double in size. Python does this too. In Python 2, and in Python 3 until 3.3, a compile-time flag determined the internal Unicode storage of the Python interpreter; a "narrow" build of the interpreter used 2-byte Unicode storage with surrogate pairs, while a "wide" build used 4-byte Unicode storage. As of Python 3.3, the internal storage of Unicode is dynamic. Python 3 source code is always parsed as UTF-8, but then as string objects are created by the interpreter their memory representation is chosen on a per-string basis, to be able to accommodate the widest code point present in the string. So Python will choose either a one-byte, two-byte, or four-byte encoding to store the string in memory, depending on what code points are present in it. This is very nice because it means iteration over a Python string is always iteration over its code points, the length of a string is always the number of code points in it, and indexing always yields the code point at index, since the internal storage of the string is fixed width and never has to include surrogates (in pre-3.3 Python, "narrow" builds would actually yield up things for which ord() gave a value in the surrogate range, and code points requiring surrogates added 2 to the length of a string rather than 1).