CycloaJS

NES EMULATOR

CycloaJS

CycloaJS is an open-source NES (Famicom) emulator implemented in JavaScript. It is a browser-playable port of a C++ project (Cycloa) and is distributed under the AGPL-3.0+ license. A live demo is hosted and the source is available on GitHub.

Jump to download ↓Jump to setup guide ↓

CycloaJS emulator screenshot.

nesPRIMARY SYSTEM

History documentedPROJECT BEGAN

Open sourcePROJECT MODEL

WHY IT MATTERS

Why CycloaJS matters

CycloaJS is primarily an educational and performance-focused port of a native C++ NES emulator into JavaScript, demonstrating how cycle-heavy emulation code can be adapted to run in modern browsers and comparing performance to other JS-based NES projects.

BEST FOR

Best use case

Embedding or testing NES ROMs in a browser environment for experimentation, demonstrations, and web-based demos of emulator performance. Not intended as a full-feature parity replacement for mature native emulators.

Gameplay from The Legend of Zelda on NES.

The people behind CycloaJS

History & provenance

CycloaJS is a JavaScript port of Cycloa, a C++ NES emulator. The repository documents that the original C++ project is available separately and that CycloaJS was created to survey JavaScript performance. The project source, README, and demo are hosted publicly; development activity and commit history are available in the GitHub repository.

What CycloaJS supports

Modern desktop browsers (Chrome, Firefox)
iPad Safari (demo notes indicate partial support)
Any platform with a standards-compliant browser and JavaScript
Browser audio: Chrome and Firefox (Web Audio API)

Download and availability

OFFICIAL DOWNLOADS

Get CycloaJS

Official source and release artifacts are published on the project's GitHub repository and Releases page. The project is AGPL-3.0+ licensed; no commercial games, BIOS, or firmware are provided.

Get CycloaJS ↗

DOCUMENTATION

Official documentation

Read the official configuration, compatibility, and troubleshooting material before changing advanced options.

Read official documentation ↗

Broken or missing download? Emulator projects move and official links sometimes change. Please contact Retro Replay so the source can be verified and updated.

CycloaJS setup guide

Setup note: This guide describes how to run CycloaJS from the official GitHub repository or use the hosted demo. CycloaJS runs in-browser and requires a web server to host files if you run it locally. CycloaJS expects NES ROM files (.nes) supplied by the user; the repository includes sample homebrew ROMs in a roms folder but does not include commercial ROMs.

Fetch the official source or use the demo: Official source: Clone or download the repository from https://github.com/ledyba/CycloaJS. For a quick test, open the hosted demo at https://app.7io.org/CycloaJS/ in a compatible browser.
Run the demo locally (recommended for development): If you want to run CycloaJS files locally, serve the repository folder with a simple HTTP server (for example: Python 3: python -m http.server 8000 in the repo root). Do not open files directly via file:// — browsers restrict features (SharedArrayBuffer, audio) when not served over HTTP/S. Once served, open http://localhost:8000/index.html (or the repository's index.html) in a supported browser.
Provide a ROM (.nes): The web UI accepts drag-and-drop of .nes files or selecting a ROM via the demo UI. Only supply ROM images you are legally permitted to use. The repository includes some sample homebrew ROMs in its roms folder; commercial games are not included.
Controls and input: Keyboard mappings are documented in the demo (A=Z, B=X, Start=Enter, Select=Space, arrow keys for D-pad). The project README and demo note that player 2 and Gamepad API support are not currently available/working.
Audio and browser compatibility: Sound output is available in Chrome and Firefox via the Web Audio API. The README and demo note tested compatibility with Chrome, Firefox, and iPad Safari; behavior and sound availability vary by browser.
Embedding CycloaJS: CycloaJS can be embedded into web pages by including the emulator files and integrating the provided HTML/JS. If embedding in other projects, follow the repository's index.html and script patterns and ensure the hosting server serves files with appropriate headers for modern browser features.
Building / development notes: The repository contains source under src/ and build artifacts. Review the README and Jakefile for any build tasks. The project is AGPL-3.0+ licensed; if you modify and distribute the project or host it as a service, comply with the AGPL terms.

Software reminder: CycloaJS is a browser-based emulator and requires user-supplied ROM files. It does not include commercial ROMs, BIOS, or firmware. Ensure you have the legal right to use any ROM you load.