js.js: A JavaScript JavaScript interpreter
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Latest commit 87cdca6 Jun 14, 2012


js.js is a JavaScript interpreter in JavaScript. Instead of trying to create an interpreter from scratch, SpiderMonkey is compiled into LLVM and then emscripten translates the output into JavaScript.


See several running demos here.


Academic paper to appear at WebApps 2012 located here.


The compiled version of js.js is 3MB and only 594KB after gzip compression. Using the Sunspider benchmark, the interpreter is about 200 times slower than Spidermonkey's native interpreter with the JIT compiler turned off. More optimizations and benchmarks are coming soon.

Example Use

Here is an example of how to use the API:

var jsObjs = JSJS.Init();
var rval = JSJS.EvaluateScript(jsObjs.cx, jsObjs.glob, "1 + 1");
var d = JSJS.ValueToNumber(jsObjs.cx, rval);
window.alert(d); //2

More examples are available in the examples directory.


The following files are located in the src directory.

File Description
js.O0.js Command-line JS shell, no optimizations
js.O0.min.js Command-line JS shell, no optimizations, closure compiled
js.O1.js Command-line JS shell, O1 optimizations
js.O1.min.js Command-line JS shell, O1 optimizations, closure compiled
js.O2.js Command-line JS shell, O2 optimizations
js.O2.min.js Command-line JS shell, O2 optimizations, closure compiled
libjs.O0.js JSAPI shared library, no optimizations
libjs.O0.min.js JSAPI shared library, no optimizations, closure compiled with js.js wrapper
libjs.O1.js JSAPI shared library, O1 optimizations
libjs.O1.min.js JSAPI shared library, O1 optimizations, closure compiled with js.js wrapper
libjs.O2.js JSAPI shared library, O2 optimizations
libjs.O2.min.js JSAPI shared library, O2 optimizations, closure compiled with js.js wrapper
jsjs-wrapper.js js.js wrapper API to make using the JSAPI easier

To run the shell, you want to run one of these:

js js.O2.min.js -e "print('hello');"
node js.O2.min.js -e "print('hello');"

To include the API in your website, include the minified libjs like this:

<script type="text/javascript" src="libjs.O2.min.js"></script>

Alternatively, you can use the wrapper script directly with a non-minified version (useful for debugging and modifications):

<script type="text/javascript" src="libjs.O2.js"></script>
<script type="text/javascript" src="jsjs-wrapper.js"></script>