Freedom251 › Multi-System Emulators
JSMoo
What JSMoo is
JSMoo is a free, MIT-licensed browser project for retro-system emulation in JavaScript and AssemblyScript. It describes current support for SNES, NES, Game Boy, Sega Master System, ZX Spectrum, and Game Gear. For SNES, the README describes cycle-accurate 65816 emulation, SPC700 emulation that is not fully cycle-accurate, and scanline-based PPU rendering; its proof of concept does not emulate DSP. Users can upload ROM files and remap controls in Settings. Sega Master System and ZX Spectrum need BIOS files, which can be uploaded under Settings/BIOS. JSMoo relies on SharedArrayBuffers and high-precision timers, so it needs special HTTP headers and cannot be started by opening index.html directly from disk. The README recommends Chrome for speed and gives a local setup using a Python 3 server script. The project also aims to help people play and understand vintage systems and processors, with learning pages planned. Its maker invites fixes and other improvements through GitHub forks and pull requests.
Who it suits
JSMoo may suit people interested in browser-based retro-system emulation or in understanding vintage systems and processors. It also invites contributors to improve the project through GitHub.
What it does well
- Describes support for six retro systems
- Users can upload ROM files and remap controls
- Distributed under the MIT License
- Project welcomes fixes and optimizations
What to know first
- Requires special HTTP headers to run
- Sega Master System and ZX Spectrum require BIOS files
- ZX Spectrum keyboard input was noted as temporarily broken
Plans
| Plan | Billed and limits | Price |
|---|---|---|
| JSMoo | MIT-licensed open-source emulator | Free |
As github.com lists them, read 11 Oct 2026. The maker's own page has the final word.
Verdict
JSMoo offers a free browser-based emulator project with several supported systems and an educational aim. Running it requires the specified server and HTTP-header setup, and some systems need BIOS files.
Facts from JSMoo's own pages
- BIOS requirement
- Sega Master System and ZX Spectrum require BIOS files, which can be uploaded under Settings/BIOS. · github.com, 11 Oct 2026
- Browser use
- JSMoo runs in a browser and the README recommends Chrome for speed. · github.com, 11 Oct 2026
- Known limitation
- The README says ZX Spectrum keyboard input was temporarily broken at the time of that commit. · github.com, 11 Oct 2026
- Learning focus
- The maker says the project aims to make vintage systems and processors easier for people to play and understand, including through planned Easy6502-like learning pages. · github.com, 11 Oct 2026
- License
- The repository says JSMoo is distributed under the MIT License. · github.com, 11 Oct 2026
- Local setup
- The README instructs users to clone the repository, run its Python 3 server script, and open the local address in Chrome. · github.com, 11 Oct 2026
- Purpose
- JSMoo is an attempt to make accurate, performant retro-system emulators in JavaScript and AssemblyScript. · github.com, 11 Oct 2026
- ROM and controls
- Users can upload ROM files and remap controls in the Settings tab. · github.com, 11 Oct 2026
- SNES emulation
- The README describes cycle-accurate 65816 emulation, not-quite-cycle-accurate SPC700 emulation, scanline-based PPU rendering, and no DSP emulation in its proof of concept. · github.com, 11 Oct 2026
- Support and contributions
- The maker invites bug fixes, optimizations, cleanup, and other improvements through GitHub forks and pull requests. · github.com, 11 Oct 2026
- Supported systems
- The project describes current support for SNES, NES, Game Boy, Sega Master System, ZX Spectrum, and Game Gear. · github.com, 11 Oct 2026
- Technical requirements
- The project uses SharedArrayBuffers and high-precision timers, so it requires special HTTP headers and cannot be run by opening index.html directly from disk. · github.com, 11 Oct 2026
Where JSMoo stands on Freedom251
- Multi-System Emulators#37 of 54