---
title: "What is EmulatorJS? An emulation engine, explained"
description: "EmulatorJS is the open-source JavaScript wrapper that lets libretro cores run in any modern browser. Here’s what it does, why it matters, and what it can and can’t emulate."
url: "https://retrogamespace.com/guides/what-is-emulatorjs"
canonical_html: "https://retrogamespace.com/guides/what-is-emulatorjs"
canonical_md: "https://retrogamespace.com/guides/what-is-emulatorjs.md"
locale: "en"
slug: "what-is-emulatorjs"
author: "Mira Köhler"
author_slug: "mira-koehler"
date_published: "2025-09-15"
date_modified: "2026-04-30"
last_reviewed: "2026-04-30"
keywords: ["EmulatorJS","libretro","WebAssembly emulation","browser emulator engine"]
source: RetroGameSpace
source_url: "https://retrogamespace.com"
---

# What is EmulatorJS? An emulation engine, explained

EmulatorJS is the open-source JavaScript wrapper that lets libretro cores run in any modern browser. Here’s what it does, why it matters, and what it can and can’t emulate.

## Definition

EmulatorJS is an open-source JavaScript front-end that wraps libretro emulator cores compiled to WebAssembly. It was first released in 2017 and is now maintained by a community of contributors on GitHub. It exposes a small, stable API — load ROM, set controls, save state, take screenshot — and hides the complexity of the underlying core.

## How it works under the hood

libretro is a project that standardizes the interface between an emulator core (the program that simulates a console’s CPU, memory, and graphics) and a front-end (the UI that wires up controls, audio, and a canvas). EmulatorJS takes the existing libretro cores written in C/C++ — for example, Snes9x for SNES, FCEUmm for NES, mGBA for Game Boy Advance — and compiles them to WebAssembly using Emscripten.

When you load a game, EmulatorJS streams the appropriate core into a Web Worker, hands it the ROM bytes, and exposes a canvas that the core renders into 60 times per second. Audio routes through Web Audio's AudioWorklet for low-latency playback. Input comes from the keyboard, on-screen touch overlay, or the Gamepad API.

## Supported systems

EmulatorJS supports a wide spread of cartridge-era and early disc-era hardware:

- Nintendo: NES, SNES, Game Boy, Game Boy Color, Game Boy Advance, Nintendo 64
- Sega: Master System, Genesis/Mega Drive, Sega CD, 32X, Saturn
- NEC: PC Engine / TurboGrafx-16, PC-FX
- Atari: 2600, 5200, 7800, Lynx, Jaguar
- Sony: PlayStation 1
- SNK: Neo Geo, Neo Geo Pocket / Color
- Other: Arcade (MAME), MSX, ColecoVision, Vectrex, WonderSwan

## What it cannot emulate (yet)

EmulatorJS is bounded by what runs comfortably in a browser. PlayStation 2, GameCube, Xbox, Wii, and 3DS are off the table — they need GPU drivers, JIT compilation that browsers restrict for security, and gigabytes of RAM. They’ll get there eventually as WebGPU and SharedArrayBuffer matures, but as of mid-2026 they remain native-only.

## EmulatorJS vs RetroArch vs custom WASM ports

RetroArch is the desktop and mobile front-end for libretro — it’s the closest analog to EmulatorJS but runs natively rather than in a browser. Custom WebAssembly ports (e.g., a hand-tuned Snes9x build) sometimes beat EmulatorJS on raw performance because they trim down the libretro abstraction layer, but they sacrifice the wide system coverage. EmulatorJS is the practical default for any browser-based site that wants to support more than two or three platforms.

## FAQ

Q: Is EmulatorJS the only browser emulator?
A: No, but it’s the most widely deployed one. Custom WebAssembly ports of individual cores exist (Snes9x, mGBA, FCEUmm), as do proprietary engines built by streaming-emulation services.

Q: Is EmulatorJS open source?
A: Yes. It’s MIT licensed and the source lives on GitHub. The libretro cores it wraps are individually licensed (most are GPL).

Q: Does EmulatorJS work offline?
A: Once the core and ROM have been loaded, the emulator runs entirely in your browser with no network calls. Closing and re-opening the tab will require re-downloading the core unless the host site uses a service worker to cache it.

---

Source: [What is EmulatorJS? An emulation engine, explained](https://retrogamespace.com/guides/what-is-emulatorjs) — Mira Köhler • Last reviewed 2026-04-30 • RetroGameSpace