| jic | Altera Quartus II JTAG Indirect Configuration |
| jil | Java in Lisp Source Code |
| jio | Join-Calculus JCC Compiled Interface |
| jl | Julia Source Code |
| jlx | JLex Code |
| jmdac | Modelio Compiled Module |
| jmk | JX Makefile |
| jmo | JMonkey Engine Model |
| jnilib | Java Native Interface Library |
| 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 |
| jrag | JastAdd Declarative Aspect |
| jrprint | JasperReports Print Document |
| jrxml | JasperReports Document |
| js | JavaScript |
| js | JavaScript Style Sheet |
| jsa | JRun Server Application |
| jsb | JAWS Script Binary File |
| jsb | JavaScript Bean |
| jsc | Rhino Compiled Javascript |
| jscript | Windows Media Player JScript |
| jsd | JAWS Script Documentation File |
| jsfl | Flash JavaScript |
| jsh | Jsh Script |
