boa_engine 0.19.0

Boa is a Javascript lexer, parser and compiler written in Rust. Currently, it has support for some of the language.


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

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Live Demo (WASM)

Try out the engine now at the live WASM playground here!

Prefer a CLI? Feel free to try out boa_cli!

Boa Crates

Boa currently publishes and actively maintains the following crates:

  • boa_ast - Boa's ECMAScript Abstract Syntax Tree
  • boa_cli - Boa's CLI && REPL implementation
  • boa_engine - Boa's implementation of ECMAScript builtin objects and execution
  • boa_gc - Boa's garbage collector
  • boa_interner - Boa's string interner
  • boa_parser - Boa's lexer and parser
  • boa_profiler - Boa's code profiler
  • boa_icu_provider - Boa's ICU4X data provider
  • boa_runtime - Boa's WebAPI features
  • boa_string - Boa's ECMAScript string implementation.

Please note: the Boa and boa_unicode crates are deprecated.

Boa Engine Example

To use Boa simply follow the below.

Add the below dependency to your Cargo.toml:

boa_engine = "0.19.0"

Then in, copy the below:

use boa_engine::{Context, Source, JsResult};

fn main() -> JsResult<()> {
  let js_code = r#"
      let two = 1 + 1;
      let definitely_not_four = two + "2";


  // Instantiate the execution context
  let mut context = Context::default();

  // Parse the source code
  let result = context.eval(Source::from_bytes(js_code))?;

  println!("{}", result.display());


Now, all that's left to do is cargo run.

Congrats! You've executed your first JavaScript using Boa!


For more information on Boa's API. Feel free to check out our documentation.

API Documentation


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 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 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 Matrix if you have any questions. Contributor discussions take place on the same Matrix Space if you're interested in contributing. We also have a Discord for any questions or issues.


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