Script written in the ExtendScript language, an extension to the JavaScript language; used for writing plug-ins for Adobe Creative Suite (CS) applications; may be created using the ExtendScript Toolkit included with Adobe Utilities.

ExtendScript is a proprietary scripting language developed by Adobe that is similar to JavaScript.

Important: Since ExtendScript supports additional functionality that JavaScript does not, JSX files are not necessarily JavaScript files. Adobe After Effects scripts also use the ".jsx" extension.