File Structure

A typical extension directory looks like this:

org.example.hello/
├─ sjmcl.ext.json
├─ icon.png
├─ frontend/
│  └─ index.js
├─ assets/
│  └─ banner.png
└─ data/

Root Directory

The extension root is the directory that contains sjmcl.ext.json.

sjmcl.ext.json

The extension manifest file. It must exist at the root.

Example:

{
  "identifier": "org.example.hello",
  "name": "Hello Extension",
  "description": "My first SJMCL extension.",
  "author": "Example Team",
  "version": "0.1.0",
  "minimalLauncherVersion": "0.8.0",
  "frontend": {
    "entry": "frontend/index.js"
  }
}

Fields:

Field	Type	Required	Description
identifier	string	yes	extension identifier
name	string	yes	extension name
description	string	no	extension description
author	string	no	author metadata
version	string	no	extension version
minimalLauncherVersion	string	no	minimum launcher version
frontend.entry	string	no	relative frontend entry path

identifier uses a namespaced format such as:

org.example.hello

Rules:

• it must contain at least one .
• each segment starts with a lowercase letter
• segments may contain lowercase letters, digits, _, and -

name must not be empty.

version uses semantic versioning, for example:

0.1.0
1.2.3

minimalLauncherVersion declares the minimum launcher version.

frontend.entry points to the frontend entry file, for example:

{
  "frontend": {
    "entry": "frontend/index.js"
  }
}

That file must call:

window.registerExtension(factory, token);

icon.png

The extension icon used by the launcher extension management page.

frontend/

The frontend entry and related scripts.

Common layout:

frontend/index.js

frontend.entry in the manifest points to this file.

assets/

Static assets such as images, audio, and video.

Access assets through api.resolveAssetUrl(path):

const logoUrl = api.resolveAssetUrl("assets/logo.png");

data/

The private runtime data directory of the extension.

This directory stores runtime files such as:

• cache
• user input
• local configuration

Extensions access it through file actions such as readFile() and writeFile().

Package Layout

Two package layouts are supported.

Layout 1: files at archive root

sjmcl.ext.json
icon.png
frontend/index.js

Layout 2: a single nested extension directory

org.example.hello/
├─ sjmcl.ext.json
├─ icon.png
└─ frontend/
   └─ index.js

Related Docs

• Extension System
• Quick Start