Apropos: http://mathiasbynens.be/notes/javascript-encoding

TL;DR:

- Javascript engines are free to internally represent strings as either UCS-2 or UTF-16. Engines that choose to go USC-2 tend to replace all glyphs outside of the BMP with the replacement char (U+FFFD). Firefox, IE, Opera, and Safari all do this (with some inconsistencies).

- However, from the point of view of the actual JS code that gets executed, strings are always UCS-2 (sort of). In UTF-16, code points outside the BMP are encoded as surrogate pairs (4 bytes). But -- if you have a Javascript string that contains such a character, it will be treated as two consecutive 2-byte characters.

  var x = '𝌆';
  x.length; // 2
  x[0];     // \uD834
  x[1];     // \uDF06

Note that if you insert said string into the DOM, it will still render correctly (you'll see a single character instead of two ?s).