| jsobj | Opera Javascript Object |
| json | JavaScript Object Notation |
| jsonld | JSON-LD Data |
| jsonp | JSON with Padding Data |
| jsr | Job Safety Report Definition |
| jsv | Java Structure Viewer Document |
| jtbackup | Just Type Backup File |
| jtk | IBM Infoprint Manager for AIX Job Ticket Data |
| jtl | Apache JMeter Data |
| jtm | React JSX Template Method Data |
| jtx | Microsoft Exchange ESE Log |
| jus | Jusan Fidelity License Data |
| jvdb | The Journal Volume Database |
| jvl | Jalview Launch Data |
| jvsk | jetVideo Skin Data |
| jvwr | JPEGView Creator Data |
| jvxl | Jmol Voxel File |
| jwb | JIRA Workflow Data |
| jwk | JSON Web Key |
| jwlibrary | JW Library Data |
| jwpub | JW Library Publication |
| jwt | JSON Web Token |
| jxe | OpenJ9 Class Loader JXE Data |
| jxl | TBC JobXML Data |
| jxt | Daqarta Juxt Data |
| jz | JamesZhu Browser Database |
| jzt | JaZwa Binary Translator Data |
| k | LS-DYNA Input Data |
| k | MacroModel Chemical Modeller Output |
| k01 | Clarion Database Keys |
