ECMAScript Harmony features in Node.js
Quick tip (via David Klassen): The following shell command lists all (highly experimental!) ECMAScript Harmony  features that can be switched on in Node.js.
node --v8-options | grep harmony
For example, Node.js 0.10.2 supports:
|--harmony_typeof||enable harmony semantics for typeof|
|--harmony_scoping||enable harmony block scoping|
|--harmony_modules||enable harmony modules (implies block scoping)|
|--harmony_proxies||enable harmony proxies|
|--harmony_collections||enable harmony collections (sets, maps, and weak maps)|
|--harmony||enable all harmony features (except typeof)|
Note: “harmony semantics for typeof” is something that (unfortunately) had to be rejected for Harmony. It would have consisted of typeof null returning 'null', but that change would break too much existing code.
- ECMAScript: ES.next versus ES 6 versus ES Harmony