| jld | Julia HDF5 Data |
| jlx | JLex Code |
| jmap | CodeWarrior JMAP Data |
| jmdac | Modelio Compiled Module |
| jmk | JX Makefile |
| jmo | JMonkey Engine Model |
| jnilib | Java Native Interface Library |
| jomproj | Visual Studio JOOB Object Model Project |
| joy | CryENGINE Facial Editor Joystick Settings |
| jpb | J Publish Manager Data |
| jpd | Java Process Definition |
| jpf | Java PathFinder Application Property Data |
| jph | JProbe Memory Snapshot |
| jpl | JProbe Settings |
| jpq | JPQ Library |
| jpt | JProbe Threadalyzer Memory Snapshot |
| jpy | JPY Data |
| jr | JR Source Code |
| jrag | JastAdd Declarative Aspect |
| jrprint | JasperReports Print Document |
| jrxml | JasperReports Document |
| js | JavaScript Style Sheet |
| js | JavaScript |
| jsa | JRun Server Application |
| jsb | JavaScript Bean |
| jsb | JAWS Script Binary File |
| jsc | Rhino Compiled Javascript |
| jsc | Jeroo Source Code |
| jscript | Windows Media Player JScript |
| jsd | JAWS Script Documentation File |
| jsdtscope | Eclipse IDE JavaScript Development Tool Scope Data |
| jsfl | Flash JavaScript |
| jsh | Jsh Script |
| jsh | JAWS Script Header |
| jsi | JAWS Personal Settings Data |