codehag / jsparagus

Parser generators

jsparagus - A JavaScript parser written in Rust

jsparagus is intended to replace the JavaScript parser in Firefox.

Current status:

• jsparagus is not on crates.io yet. The AST design is not stable enough. We do have a build of the JS shell that includes jsparagus as an option (falling back on C++ for features jsparagus doesn't support). See mozilla-spidermonkey/rust-frontend.

• It can parse a lot of JS scripts, and will eventually be able to parse everything. See the current limitations below, or our GitHub issues.

• Our immediate goal is to support parsing everything in Mozilla's JS test suite and the features in test262 that Firefox already supports.

Join us on Discord: https://discord.gg/tUFFk9Y

Getting started

make init
make all

(Note: This takes about 3 minutes to run on my laptop. Part of jsparagus is generated by a Python script, and the script is pretty slow. We're working on it!)

When it's done, you can:

• Run make check to make sure things are working.

• cd crates/driver && cargo run to test the JS parser and bytecode emitter.

Limitations

It's all limitations, but I'll try to list the ones that are relevant to parsing JS.

• Lookahead assertions are limited to one token. (The JS grammar contains an occasional [lookahead != `let [`] and even [lookahead != `async [no LineTerminator here] function`].)

• Error messages are poor.

• No table compaction or table optimization. There's plenty of low-hanging fruit there.