jsparagus's Introduction

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.

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