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# Custom classes

An extension's classes are ordinary Jx `.class.json` descriptors: JSON Schema 2020-12 documents describing a class's constructor parameters, fields, and methods, plus an optional `$implementation` key pointing at the JavaScript module that implements it. Hosts read the descriptor to learn everything about the class; they import the implementation only to invoke it.

Each class named in the [manifest](/docs/extending/extensions/anatomy)'s `classes` map becomes a `$prototype`-visible name in any project that enables the extension. A page can then use it as state with no `$src`:

```json shiki
```

## A descriptor at a glance

The parser's `Markdown.class.json`, heavily abbreviated:

```json shiki
```

The `$defs` object is organized into well-known categories:

| Category      | Purpose                                                                  |
| ------------- | ------------------------------------------------------------------------ |
| `parameters`  | Constructor parameter properties (config object fields)                  |
| `fields`      | Instance fields (private if `#`-prefixed)                                |
| `constructor` | Constructor body and super args                                          |
| `methods`     | Instance methods and accessors, plus static capability methods by `role` |
| `returnTypes` | Named return-type schemas for tooling                                    |

## The external class contract

Every class, extension-shipped or project-local, satisfies the same runtime contract:

- **Constructor** receives a single `config` object containing all `state` properties except the reserved keywords (`$prototype`, `$src`, `$export`, `timing`, `default`, `description`). For the state entry above, `config` is `{ src: "./content/about.md" }`.
- **Value resolution** is checked in order: `instance.resolve()` (async, awaited) → `instance.value` (getter or property) → the instance itself.
- **Reactivity is optional**: implement `subscribe(callback)` / `unsubscribe()` and the runtime re-renders when you notify.

Methods declare their output shape with a JSON Schema in `returnType` (often a `$ref` into `$defs/returnTypes`). Tooling uses this to reason about a class without executing it. A `resolve` whose `returnType` is an array marks instances as valid sources for mapped iteration, so Studio offers the class in repeater pickers.

## `$implementation`

`$implementation` is resolved relative to the `.class.json` file and names the module exporting the class; the export is named by the descriptor's `title`. The parser ships TypeScript in `src/` and points `$implementation` at the built neighbor (`./markdown.js`).

When `$implementation` is **absent**, the class is self-contained: the compiler generates an ES class from the schema itself. Constructor (with `super()` support), private `#fields`, getters/setters, async methods, and the `extends` clause all come from the descriptor. This is how small classes ship with no JavaScript at all; structured `body` statements are covered in [Functions](/docs/framework/concepts/functions).

## Admission blocks

A plain class with nothing but the contract above is a state prototype: construct, `resolve()`, done. A class additionally participates in **host dispatch** through one or more top-level admission blocks:

| Block       | Grants                                                                                      | Covered in                                                      |
| ----------- | ------------------------------------------------------------------------------------------- | --------------------------------------------------------------- |
| `format`    | File-extension dispatch: parsing, serialization, content discovery/loading, Studio editing. | [Custom formats](/docs/extending/extensions/formats)            |
| `project`   | Ownership of a project.json section: data loading, `$paths` resolution, Studio settings UI. | [Project sections](/docs/extending/extensions/project-sections) |
| `server`    | A mounted route subtree in the deployed-site worker and the dev server.                     | [Server mounts](/docs/extending/extensions/server)              |
| `connector` | A database connection provider: dialect, schema deploy, bindings, connection testing.       | [Connectors](/docs/extending/extensions/connectors)             |

`Markdown` carries a `format` block; the connector's `Data` class carries `project`**and**`server` (it owns the `data` section and mounts `/_jx/data`). Blocks compose freely on one class.

## How hosts introspect

Hosts follow a strict JSON-first contract, and the registry (`buildExtensionRegistry()` in `@jxsuite/schema/extension-registry`) implements it once for node and browser hosts:

1. Resolve each `extensions` entry to a package root; read the manifest named by its `"jx"` field; read each listed `.class.json`.
2. Detect participation via the admission blocks.
3. Find capabilities by scanning `$defs.methods` for well-known `role` values; the method's `identifier` (fallback: its key) names the static method.
4. To invoke: import `$implementation`, take the export named by the class `title`, call `Export[identifier](...args)`.
5. Respect `timing`: if the host's environment is not listed, delegate to the dev server.

Step 4 is the only point where extension code runs in a host. Everything Studio shows comes from steps 1–3: settings forms, format icons, capability option UIs.

## Capability methods

Capabilities are the static methods step 3 discovers: `parse`, `load`, `projectData`, `mount`, `lower`, and the rest of the well-known roles. Their contract (timing, options-as-parameters, and the compile-away `lower` hook) is the subject of [Capability methods](/docs/extending/extensions/capabilities).

## Related

- [Capability methods](/docs/extending/extensions/capabilities): the static method contract in depth
- [Custom formats](/docs/extending/extensions/formats): the `format` admission block
- [Data prototypes](/docs/framework/concepts/data-prototypes): the built-in classes yours sit beside
- [Tutorial: a TOML format extension](/docs/extending/extensions/tutorial-toml-format): a full descriptor built from scratch

✕

Start here[Overview](/docs/start/)Get started[Install Jx Studio](/docs/start/install/)[Your first project](/docs/start/first-project/)[A tour of Jx Studio](/docs/start/studio-tour/)Tutorials[Tutorial: interactive component](/docs/start/first-component/)[Tutorial: blog with collections](/docs/start/first-collection/)[Video tutorials](/docs/start/videos/)Coming from another tool[Coming from WordPress](/docs/start/coming-from-wordpress/)[Coming from Webflow or Framer](/docs/start/coming-from-webflow/)[Coming from Wix or Squarespace](/docs/start/coming-from-wix/)[Coming from Lovable or Builder.io](/docs/start/coming-from-ai-builders/)Studio[Overview](/docs/studio/)[The desktop app](/docs/studio/desktop/)The workspace[Overview](/docs/studio/interface/)[Welcome screen](/docs/studio/interface/welcome-screen/)[Editors and views](/docs/studio/interface/modes/)[The canvas](/docs/studio/interface/canvas/)[Documents and panes](/docs/studio/interface/tabs/)[Quick 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architecture[Overview](/docs/framework/site/)[project.json](/docs/framework/site/project-json/)[Routing](/docs/framework/site/routing/)[Layouts](/docs/framework/site/layouts/)[Content collections](/docs/framework/site/content-collections/)[Jx Markdown](/docs/framework/site/jx-markdown/)[Obsidian vault as content](/docs/framework/site/obsidian-vault/)[Relationships](/docs/framework/site/relationships/)[SEO and metadata](/docs/framework/site/seo/)[Locales and languages](/docs/framework/site/i18n/)[Images](/docs/framework/site/images/)[Redirects](/docs/framework/site/redirects/)[Feeds](/docs/framework/site/feeds/)[Site search](/docs/framework/site/search/)[Build output and adapters](/docs/framework/site/deployment/)How compilation works[Overview](/docs/framework/build/)[The dev server](/docs/framework/build/dev-server/)[CLI commands](/docs/framework/build/cli/)Working with agents[Overview](/docs/framework/agents/)[Authoring rules for 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guestbook](/docs/extending/extensions/tutorial-guestbook/)[First-party extensions](/docs/extending/extensions/first-party/)Embedding[Overview](/docs/extending/embedding/)[Hosting the Studio assets](/docs/extending/embedding/hosting/)[Writing a platform adapter](/docs/extending/embedding/platform-adapter/)[The backend protocol](/docs/extending/embedding/backend-protocol/)[Dev server internals](/docs/extending/embedding/dev-server/)[Embedding the runtime in your app](/docs/extending/embedding/runtime-host/)[Writing assistant tools](/docs/extending/embedding/assistant-harness/)UI kit[The Jx UI kit](/docs/extending/ui-kit/)Reference[Protocol routes](/docs/extending/reference/studio-routes/)[Implementation status](/docs/extending/reference/implementation-status/)[Spec changelog](/docs/extending/reference/spec-changelog/)[Standards alignment](/docs/extending/reference/standards/)Contributing[Working in the monorepo](/docs/extending/contributing/monorepo/)[Contributing to these docs](/docs/extending/contributing/docs/)[AI assistant evals](/docs/extending/contributing/ai-evals/)

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