| jslib | Unity WebGL Extension |
| jsm | JAWS Script Message File |
| jsm | JavaScript Module |
| jsp | Java Servlet |
| jspa | Java Servlet Alias |
| jspf | Java Server Page Fragment |
| jspre | Unity WebGL Extension |
| jspx | Java Servlet XML |
| jss | JavaScript Style Sheet |
| jss | JAWS Script Source |
| jst | Code Crusader Symbol Database |
| jsx | Adobe ExtendScript File |
| jsxbin | Adobe ExtendScript Binary File |
| jsxinc | Adobe ExtendScript Include File |
| jtb | JavaCC Grammar File |
| jtm | JT Harness Template Data |
| jtpl | IBM Rational Application Developer for WebSphere Template Data |
| jtr | JavaTest Results Data |
| ju | JU Source Code |
| judo | JudoScript Source Code |
| jug | Jug Code |
| jui | QT Jambi User Interface Data |
| juk | Juka Source Code |
| jv | ICE JVCC Source Code |
| jwg | OpenScript Script |
| jwp | CloudTran Build Properties |
| jwr | Rocket LegaSuite GUI Runtime |
| jws | Java Web Service |
| jx | Cocoon JX Template |
| jxs | Jitter Shader Description Data |