Node.js v0.12 is finally here! But the incubation period has been particularly long, with the v0.11 branch diverging significantly from what we have been used to in v0.10. While serious attempts have been made to keep the Node.js API stable, there are some major changes that will cause upgrade pain for some. Most notably:

Streams3: the hybrid old/new approach of Streams2 has gone and we now simply have "flowing" and "not flowing" streams. While the external API should remain consistent, upgrade pain may come in the form of edge-cases.

HTTP Agent: instead of the somewhat arbitrary limit of 5 simultaneous connections per host for the HTTP client, the limit is now Infinity, which should improve performance and predictability. Unfortunately, heavy HTTP client users will likely run in to new errors when hitting operating system limits.

V8: while not obvious, changes to the way V8 executes your code may leave you with unexpected behaviour. Specifically, the performance profile will have changed and changes to the garbage collector may leave you with unexpected performance changes, memory use changes and even expose some bugs previously hidden due to assumptions of how the garbage collector works (this is particularly true for native addons).

Native (C++) Addons: here lies the biggest pain for upgrades to Node.js v0.12. During the v0.11 development cycle, V8 had a major overhaul of its C++ API. These significant changes were the inspiration for the NAN project which provides a compatibility layer between different versions of V8.