Mathematica has several representations for the code the main one is the inputForm, the thing that users type in, and there is a fullForm which is the full ast, and there is a way to manipulate it to create new functions on the fly (or really any other objects, like graphic or sound, since everything in Mathematica is represented in a uniform way)

For instance if in input form you have {1 + 1, 2 *3 }, in fullForm this becomes List[Plus[1,1], Times[2, 3]], which is the readable version of lisp and internally represented the same way [List [Plus 1 1] [Times 2 3]].

The fullForm is accessible from the language, and can be manipulated using standard language feature, kind of like eval but better.

It would be really cool to have something like this in javascript, but unfortunately looks like javascript tools tend to create nonuniform ast, that is hard to traverse and manipulate

For the moment, we're not trying to make the AST visible to the language although this would definitely be interesting.

The "recommended" manner to manipulate the AST these days is to use e.g. Babel/Babylon or Esprima. I realize that it's not as integrated as what you have in mind, of course, but who knows, maybe a further proposal one of these days?