Reflect.parse takes an optional `builder` argument. If present, it's an object with a ton of methods, and it's used to build the resulting AST. (The default, if `builder` is missing, is to build a straightforward JSON AST.) https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Parser_API#Builder_objects This API is brittle. Every time we add anything to JS syntax--or remove an obsolete feature--all code using the builder API breaks. Therefore I doubt anyone is using it and we should remove it.