ECMAScript Interpreter
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mjs is an interpreter for ECMAScript 1st edition (a.k.a. Javascript 1997) written in C++17. It was written as a hobby project for the purpose of gaining a better understanding of how Javascript actually works. As such, it neither strives to be fast, secure nor particularly beautiful. In fact, some things are downright terribly implemented; also no real programs were actually tested.

Most of ES1 should be implemented (to some degree...), but see the TODO file for details. Also be aware that you probably haven't programmed in ES1 before and will find most modern conveniences (e.g. array and object literals) missing from the specification.

Support for ECMAScript 3rd edition is currently (slowly) under way.


You need a modern C++ compiler that supports C++17 and CMake 3.7 or later. It has currently been tested with the following compilers:

  • Visual C++ 2017 15.8.8 on Windows 10
  • GCC 7.1.0 (x64 distribution from Stephan T. Lavavej) on Windows 10
  • GCC 8.2.0 (on x64 Linux, Debian testing as of October 2018)

To run the tests build the check target.

The interpreter is built in the src directory. It accepts a Javascript file on the command line or starts in REPL mode if no argument is given.

These steps should work for most people:

mkdir build
cd build
cmake .. # Here you may want to use e.g. CXX=g++-8 cmake ..
cmake --build .
cmake --build . --target check


The project is currently light on the documentation front, but here is what's available so far:


Forget about it :) Feel free to raise an issue on GitHub, but don't expect anything to come of it.