7 unstable releases

0.17.3 Sep 26, 2023
0.17.2 Sep 26, 2023
0.17.0 Jul 8, 2023
0.16.0 Sep 25, 2022
0.14.0 Mar 15, 2022

#74 in Data structures

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14,508 downloads per month
Used in 13 crates (5 directly)




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_interner is a string interner for compiler performance.

Crate Overview

The idea behind using a string interner is that in most of the code, strings such as identifiers and literals are often repeated. This causes extra burden when comparing them and storing them. A string interner stores a unique usize symbol for each string, making sure that there are no duplicates. This makes it much easier to compare, since it's just comparing to usize, and also it's easier to store, since instead of a heap-allocated string, you only need to store a usize. This reduces memory consumption and improves performance in the compiler.


~45K SLoC