[hadcomplained]

IMHO what is essential as a language feature for writing interpreters/compilers is algebraic data types and pattern matching on them, because they make things much easier when manipulating ASTs, which is a frequent task in implementing these programs.

Unfortunately besides functional programming languages (Haskell, OCaml, etc) or modern languages (such as Rust), there are few having these features, and JavaScript is no exception. Of course you can use a poor man's substitute such as the Visitor pattern, but it's quite a hassle without a direct language support.

[DmitrySoshnikov]

Yes, good point on pattern matching and algebraic data types (initially I was actually considering Rust for implementation, but not OCaml). The visitor pattern is a classic though, and is transferable knowledge further, to process different kinds of graphs, trees, etc. BTW, JavaScript will likely be getting pattern matching soon as well.