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KJS: A Complete Formal Semantics of JavaScript

KJS is the most complete and throughly tested formal semantics of JavaScript to date. Being executable, KJS has been tested against the ECMAScript conformance test suite, and passes all 2,782 core language tests.

In addition to a reference implementation for JavaScript, KJS also yields a semantic coverage metric for a test suite: the set of semantic rules it exercises. See test262-coverage for details.

Being symbolically executable, KJS can also be used for formal analysis and verification of JavaScript programs. This is demonstrated by verifying non-trivial programs (verification), and finding known security vulnerabilities (security-attack).

How to Run Semantics

The following instructions are for standard Debian/Ubuntu distributions, especially for Ubuntu 14.04 LTS 64-bit.

0. Install Basic Dependencies

Install JDK 1.8:

$ sudo add-apt-repository ppa:webupd8team/java
$ sudo apt-get update
$ sudo apt-get install oracle-java8-installer

Install build-essential, git, and maven:

$ sudo apt-get install build-essential
$ sudo apt-get install git
$ sudo apt-get install maven

1. Install K

This semantics is compatible with a customized version of the latest K framework. You can install the version of K as follows:

$ git clone
$ cd k
$ git checkout -b kjs origin/kjs
$ mvn package

Ensure kompile and krun are included in your $PATH:

$ export PATH=$PATH:<path-to-k>/k-distribution/target/release/k/bin

2. Install Node.js

We use Node.js implementation for Math.sin, Number.toFixed, and Number.toString to test programs modulo the unsupported libraries.

$ sudo apt-get install nodejs
$ (cd $(dirname `which nodejs`); sudo ln -s nodejs node)

3. Install KJS

You can compile the semantics using Makefile:

$ make

4. Run KJS

You can run a JavaScript program using

$ ./ <your-javascript-program>.js

For a 'hello-world' example,

$ cat >hello-world.js <<EOL
console.log("hello world!");

$ ./ hello-world.js
hello world!

Note that, however, KJS support only a part of standard libraries, and may fail to run a program with unsupported libraries such as Math, Date, RegExp, and JSON. For example, KJS fails to run the following program, getting stuck at the unsupported library function call:

$ cat >time.js <<EOL

$ ./ time.js
Error: failed to run the program: time.js
Check the dumped output: /tmp/kjs.bd7uNgkKud.out

$ cat /tmp/kjs.bd7uNgkKud.out
        Call ( @ ( "" ) , @DateOid , @Nil ) ~> Exit ;

5. Run ECMAScript conformance test suite(test262)

You can run the core ECMAScript conformance test suite (2,782 tests in total) as follows: (where N is a number of parallel processes to be used)

$ make -k -j N test262-core

Running all the tests will take 2 hours using 4 parallel processes in a machine with Intel Xeon CPU 3.40GHz and DDR3 RAM 8GB 1600MHz.

You can also selectively run a part of the tests by using the environment variables:

$ TEST262_CORE_POSITIVE=<list-of-positive-tests> make test262-core-positive
$ TEST262_CORE_NEGATIVE=<list-of-negative-tests> make test262-core-negative

Test Result of test262

We provide a test result of the core test262, test262.out. For each test, it reports succeed when passed the test, and failed when failed.

Note that there are two types of tests: positive and negative tests. A negative test is identified by @negative in its preamble. The negative tests should fail to run.

Invalid Tests

Among the 2,782 core tests, our semantics reports parsing errors for the following 6 programs, which is the correct behavior according to the language standard ECMAScript 5.1 specification. These programs have function declarations inside a local block such as try or while loop, while function declaration is only allowed at top-level (refer to the grammar specification Annex A.5 Functions and Programs). This grammar mismatch problem was already admitted by the standard committee, and will be corrected in the next standard: Draft Specification of Ecma-262 Edition 6.


Note that we consider the above tests as negative tests, such that it will report succeed.

Built-in Objects Support

Currently, KJS supports the standard built-in objects as follows:

Directory Structure

Note: A virtual machine, in which all of the required programs and libraries have been installed and configured, is provided at:


KJS: A Complete Formal Semantics of JavaScript






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