It allows users to interactively move through parameter space of all the code constants, without modifying anything in code.
You can find more details (including info on AST transformations) in this blog post: building parametrium.
Parametrium is a parameter space explorer for P5.js sketches. It analyses abstract syntax tree to find constants in uploaded code, and modifies them with interactive evolutionary algorithm. It was built during my 1-project-a-month meta-project in March 2017.
You can play with live version here: http://szymonkaliski.github.io/parametrium.
Code is open sourced and available with MIT license here: szymonkaliski/parametrium.