There is a wonderful lecture by in the Discovery Summit Series:
Using JSL to Develop Efficient, Robust Applications (EU 2018 415)
Based on the content of Expression Handling Functions: Part I - Unraveling the Expr(), NameExpr(), Eval(), ... Conundrum
@joseph_morgan explains the secrets and possibilities of Expression Handling in JSL - and the trapdoors associated with it.
Some info can be found in the Scripting Guide: advanced-expressions-macros-and-lists