| ji | Join-Calculus Interface Source Code |
| jic | Altera Quartus II JTAG Indirect Configuration |
| jil | Java in Lisp Source Code |
| jin | Java Servlet Include |
| jio | Join-Calculus JCC Compiled Interface |
| jjs | Nashorn JJS Script |
| jl | Julia Source Code |
| jl | Sawfish Lisp-based Script Source Code |
| 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 |
| jop | JOP JOPizer Generated Bytecode |
| joy | CryENGINE Facial Editor Joystick Settings |
| jpage | Eclipse IDE Java Scrapbook Page |
| 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 |
| js | JavaScript Style Sheet |
| js1 | Unity Javascript |
| jsa | JRun Server Application |
