AcornJS

Acorn is a tiny, fast JavaScript parser, written completely in JavaScript. Acorn is designed to support plugins which can, within reasonable bounds, redefine the way the parser works. Plugins can add new token types and new tokenizer contexts (if necessary), and extend methods in the parser object. This is not a clean, elegant API—using it requires an understanding of Acorn's internals, and plugins are likely to break whenever those internals are significantly changed. But still, it is possible, in this way, to create parsers for JavaScript dialects without forking all of Acorn. And in principle it is even possible to combine such plugins, so that if you have, for example, a plugin for parsing types and a plugin for parsing JSX-style XML literals, you could load them both and parse code with both JSX tags and types.