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JavaScript strings – UTF-16 vs UCS-2?

I’ve read in some places that JavaScript strings are UTF-16, and in other places they’re UCS-2. I did some searching around to try to figure out the difference and found this:

Q: What is the difference between UCS-2 and UTF-16?

A: UCS-2 is obsolete terminology which refers to a Unicode implementation up to Unicode 1.1, before surrogate code points and UTF-16 were added to Version 2.0 of the standard. This term should now be avoided.

UCS-2 does not define a distinct data format, because UTF-16 and UCS-2 are identical for purposes of data exchange. Both are 16-bit, and have exactly the same code unit representation.

Sometimes in the past an implementation has been labeled “UCS-2” to indicate that it does not support supplementary characters and doesn’t interpret pairs of surrogate code points as characters. Such an implementation would not handle processing of character properties, code point boundaries, collation, etc. for supplementary characters.


So my question is, is it because the JavaScript string object’s methods and indexes act on 16-bit data values instead of characters what make some people consider it UCS-2? And if so, would a JavaScript string object oriented around characters instead of 16-bit data chunks be considered UTF-16? Or is there something else I’m missing?

Edit: As requested, here are some sources saying JavaScript strings are UCS-2:

EDIT: For anyone who may come across this, be sure to check out this link:



JavaScript, strictly speaking, ECMAScript, pre-dates Unicode 2.0, so in some cases you may find references to UCS-2 simply because that was correct at the time the reference was written. Can you point us to specific citations of JavaScript being “UCS-2”?

Specifications for ECMAScript versions 3 and 5 at least both explicitly declare a String to be a collection of unsigned 16-bit integers and that if those integer values are meant to represent textual data, then they are UTF-16 code units. See

EDIT: I’m no longer sure my answer is entirely correct. See the excellent article mentioned above, which in essence says that while a JavaScript engine may use UTF-16 internally, and most do, the language itself effectively exposes those characters as if they were UCS-2.

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