Use this template to understand how the Code Action actually works.
I hope this is self-explanatory, while we'll be out with the documentation.
The basics are:
The language is actually Javascript.
Use context.attributes.KEY or context.attributes[KEY] to read the value of a flow attribute
Ex.
let age = context.attributes.age;
Use context.setAttribute(KEY, VALUE) to set the value of an attribute
Ex.
context.setAttribute('age', 35);