PDF files can contain Javascript fragments to perform a variety of tasks including validation of form input data. To accommodate this, MuPDF has its own Javascript engine, ‘mujs’.
The run subtool of mutool can be used to execute Javascript using this engine. While running such scripts, the engine provides a Javascript reflection of the C API (actually, more correctly a reflection of the object-oriented version of the API as found in C++).
This enables the scripting of very powerful tasks, including opening, manipulating, rendering and resaving files. Some examples of such scripts can be found in docs/examples.