4 releases

0.17.3 Sep 26, 2023
0.17.2 Sep 26, 2023
0.17.1 Sep 26, 2023
0.17.0 Jul 8, 2023

#308 in Parser implementations

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12,931 downloads per month
Used in 9 crates (3 directly)


12K SLoC


Boa Logo

This is an experimental Javascript lexer, parser and interpreter written in Rust. Currently, it has support for some of the language.

Build Status codecov Crates.io Docs.rs Discord

Live Demo (WASM)


You can get more verbose errors when running from the command line.


To know how much of the ECMAScript specification does Boa cover, you can check out results running the ECMASCript Test262 test suite here.


Please, check the CONTRIBUTING.md file to know how to contribute in the project. You will need Rust installed and an editor. We have some configurations ready for VSCode.


Check debugging.md for more info on debugging.

Web Assembly

This interpreter can be exposed to JavaScript! You can build the example locally with:

npm run build

In the console you can use window.evaluate to pass JavaScript in. To develop on the web assembly side you can run:

npm run serve

then go to http://localhost:8080.


  • Clone this repo.
  • Run with cargo run -- test.js in the project root directory where test.js is a path to an existing JS file with any valid JS code.
  • If any JS doesn't work then it's a bug. Please raise an issue!



Command-line Options

Usage: boa [OPTIONS] [FILE]...

  [FILE]...  The JavaScript file(s) to be evaluated

      --strict                        Run in strict mode
  -a, --dump-ast [<FORMAT>]           Dump the AST to stdout with the given format [possible values: debug, json, json-pretty]
  -t, --trace                         Dump the AST to stdout with the given format
      --vi                            Use vi mode in the REPL
  -O, --optimize
      --flowgraph [<FORMAT>]          Generate instruction flowgraph. Default is Graphviz [possible values: graphviz, mermaid]
      --flowgraph-direction <FORMAT>  Specifies the direction of the flowgraph. Default is top-top-bottom [possible values: top-to-bottom, bottom-to-top, left-to-right, right-to-left]
      --debug-object                  Inject debugging object `$boa`
  -m, --module                        Treats the input files as modules
  -r, --root <ROOT>                   Root path from where the module resolver will try to load the modules [default: .]
  -h, --help                          Print help (see more with '--help')
  -V, --version                       Print version


See Milestones.


See Benchmarks.


See Profiling.




Feel free to contact us on Discord.


This project is licensed under the Unlicense or MIT licenses, at your option.


Boa's boa_ast crate implements an ECMAScript abstract syntax tree.

Crate Overview

boa_ast contains representations of Parse Nodes as defined by the ECMAScript spec. Some Parse Nodes are not represented by Boa's AST, because a lot of grammar productions are only used to throw Early Errors, and don't influence the evaluation of the AST itself.

Boa's AST is mainly split in three main components: Declarations, Expressions and Statements, with StatementList being the primordial Parse Node that combines all of them to create a proper AST.


~60K SLoC