I dare not try and reexplain the discussion in this bug report as my understanding feels insufficient, but the entire discussion at http://code.google.com/p/v8/issues/detail?id=761#c14 (note, I've linked to the 14th commment in the discussion, but there's more interesting stuff above) talks about it. At the core is a distinction between v8's internal representation of strings and it's API vs. what a browser engine which embeds v8 might do.