mconca / 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

Part of jsparagus is generated by a Python script, so start by setting up your Python environment, make sure cargo-fmt is installed, then use make all to build the parser:

python3 -m venv venv
source venv/bin/activate
pip install --upgrade pip
pip install -r requirements.txt

cargo install cargo-fmt

git config core.hooksPath .githooks

make all

(Note: This takes about 3 minutes to run on my laptop. The Python script is slow. We're working on it!)

When it's done, you can:

• Run make check to make sure things are working.

• cd rust && 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`].)

• Restricted productions ("[no LineTerminator here]") are not implemented yet.

• Error messages are poor.

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