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- dependency-name: mocha
  dependency-type: direct:development
  update-type: version-update:semver-major

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A JavaScript NES emulator.

It's a library that works in both the browser and Node.js. The browser UI is available at


For Node.js or Webpack:

$ npm install jsnes

(Or yarn add jsnes.)

In the browser, you can use unpkg:

<script type="text/javascript" src=""></script>


// Initialize and set up outputs
var nes = new jsnes.NES({
  onFrame: function(frameBuffer) {
    // ... write frameBuffer to screen
  onAudioSample: function(left, right) {
    // ... play audio sample

// Read ROM data from disk (using Node.js APIs, for the sake of this example)
const fs = require('fs');
var romData = fs.readFileSync('path/to/rom.nes', {encoding: 'binary'});

// Load ROM data as a string or byte array

// Run frames at 60 fps, or as fast as you can.
// You are responsible for reliable timing as best you can on your platform.
// ...

// Hook up whatever input device you have to the controller.
nes.buttonDown(1, jsnes.Controller.BUTTON_A);
nes.buttonUp(1, jsnes.Controller.BUTTON_A);
// ...


To build a distribution:

$ yarn run build

This will create dist/jsnes.min.js.

Running tests

$ yarn test

Embedding JSNES in a web page

You can use JSNES to embed a playable version of a ROM in a web page. This is handy if you are a homebrew ROM developer and want to put a playable version of your ROM on its web page.

The best implementation is jsnes-web but unfortunately it is not trivial to reuse the code. You'll have to copy and paste the code from that repository, the use the <Emulator> React component. Here is a usage example..

A project for potential contributors (hello!): jsnes-web should be reusable and on NPM! It just needs compiling and bundling.

A more basic example is in the example/ directory of this repository. Unfortunately this is known to be flawed, and doesn't do timing and sound as well as jsnes-web.

Formatting code

All code must conform to Prettier formatting. The test suite won't pass unless it does.

To automatically format all your code, run:

$ yarn run format


JSNES is based on James Sanders' vNES, and owes an awful lot to it. It also wouldn't have happened without Matt Wescott's JSSpeccy, which sparked the original idea. (Ben, circa 2008: "Hmm, I wonder what else could run in a browser?!")