On http://www.eclipse.org/webtools/jsdt/ it says that the most recent version is 3.4. My Eclipse version is 4.2 Juno.
In Help → Install new software... when I search for it I only get the 1.4 JSDT. I've got the following software sites enabled:
Juno http://download.eclipse.org/releases/juno Enabled
Mylyn for Eclipse Juno http://download.eclipse.org/mylyn/releases/juno Enabled
The Eclipse Project Updates http://download.eclipse.org/eclipse/updates/4.2 Enabled
So how do I install the 3.4 JSDT?
JSDT was only released under Web Tools Platform (WTP) version 3.4, this doesn't mean that the version of the JSDT itself has version 3.4 too.
You could probably update your WTP Version to 3.4. The repository could also be found under http://download.eclipse.org/webtools/repository/juno