ExtendScript is one of the scripting languages available for implementing custom functionality for Adobe products like Photoshop, Illustrator, etc. ExtendScript is based on, and has been kept locked specifically to, ECMAScript 3 (released in 1999).
ExtendScript, or JSX (not to be confused with React's JSX markup) is a scripting language based on ECMAScript 3 (1999) used for scripting custom functionality in Adobe products like Photoshop, Illustrator, etc.
It is syntactically equivalent to JavaScript 1.5, with the same standard library, as well as a custom API for interacting with Adobe product applications and documents.
ExtendScript does not support any language features or standard library functions that were introduced after ECMAScript 3.