A simple jasmine + mesen-based test runner for the NES. Validate that your generated roms have the proper size/signature, and do the right things when you run them.

Note: This is a very early project! If you find this thing useful, please let me know! If I see people using this I'll be much more likely to carry the project forward.


  • Write unit tests using jasmine and javascript
  • Inspect memory locations during game execution
  • Use labels and variables defined in assembly/C when debug information is configured
  • Compare screenshots with expected screenshots
  • Inspect rom file for correctness (header is present, rom length matches header)
  • Support for both windows and linux! (Mac support will come if mesen is ever officially supported there)
  • Distributed as a single binary, or an npm module

Quick Links

Example tests

More examples can be found in the examples/ folder!

it('Validates that this is a valid NES rom', async () => {
    // Rom paths are relative to the test script
    romData = new NesRomFile('./data/working-nrom.nes');

it('gameState C variable starts at 0, then is updated after you dismiss the menu', async () => {

    // Create a new emulator instance
    emulator = new NesEmulator('./data/working-nrom.nes');
    // Open the emulator

    // Check the initial state of gameState
    expect(await emulator.getByteValue('gameState')).toEqual(0)

    // Wait for input to start being accepted, then press start
    await emulator.runCpuFrames(60);
    await emulator.sendInput({start: true});
    await emulator.runCpuFrames(30);

    // Validate gameState was properly updated
    expect(await emulator.getByteValue('gameState')).toEqual(11);

    // Stop the emulator

There are plenty of examples in the examples/ folder.

What if a test fails?

You will get a list of the failed tests, along with a line number and reason for failure.

Test Results


Download the binary for your operating system from the github releases page.

Put this somewhere on your system, then create a folder for your tests. (You can also copy the examples folder from this repo to start!)

Run the program by calling it on the command line, with the relative path to your tests.

./nes-test-win ./examples

Major rewrite version 0.3

0.3 is a complete rewrite of the features of this library. Old code will not work! 0.3 restructures the library to allow better passing of data to/from the emulator. It also allows much more intuitive test writing. It also installs Mesen automatically, so no manual setup is required!

If you need 0.2, here is a link:

0.2 version

Special notes for linux

This program will work as expected with linux, however Mesen has a few special requirements to get it going. Namely, you'll need the following packages, which may be installed using apt or a similar package manager.

  • mono-complete
  • libsdl2-2.0
  • gnome-themes-standard (You might be able to get away without this one if you only ever use the test runner)

If you run into issues, try downloading a copy of Mesen manually and getting it to work. The tool should inherit any changes you make to your system.

Find more information on running Mesen on linux here:

Otherwise follow all of the instructions in the above section. Mesen will be automatically downloaded in a config directory when you first run the tests.


There are plenty of examples with documentation in the examples folder


All API methods are documented on the github pages site


There is not much configuration for this module yet, but a few environment variables can customize behavior to help your debugging:

  • DEBUG_OPEN_MESEN - Set this to true to open mesen and run a test in the script runner. (Note: enable a single test with fit() or behavior may be unpredictable!)

Gotchas with jasmine

  • Tests in jasmine time out after 5 seconds by default. If you need longer, you can set the DEFAULT_TIMEOUT_INTERVAL environment variable when running the tool. (Values are in milliseconds)
  • The NesEmulator class methods are almost all asynchronous, so be sure to prefix them with await, or you could encounter weird bugs.

Potential future features

  • Allow installs as a node module. (Note: this might work now, if you're adventurous feel free to try!)
  • Better test roms, with source
  • Instructions for running this in continuous integration environments
    • The short version is, I probably need to publish to npm, then have users run npx nes-test
    • For now you could download the binary directly and run it as usual


Free to email me or create Github issues.

I want to hear from you! Also if you'd like to contribute, submit an issue or a PR! I don't have many contribution guidelines (yet!) but if you've got something you want to see, I'm all ears.