Meta:Magical allows you to annotate objects in JavaScript, and inspect them in an interactive environment (REPL).
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Meta:Magical Chat on Gitter Build status Licence: MIT Stability: Experimental

This project is still in early stages of development.

Meta:Magical is a project to improve documentation tooling in JavaScript. At its core, Meta:Magical allows one to annotate runtime objects with arbitrary data, and query this data later.

This allows for some very interesting features. For example:

  • A REPL may use this data to provide auto-completion features, documentation, or even show related functionality.

  • A tool may use example code attached to an object and run them as test cases automatically.

  • A compiler may output annotated objects, and enable people to inspect these annotations in ways that aren't possible with static documentation generators due to the dynamic nature of JavaScript.

The current focus of development is to provide a robust framework for annotating live objects, and an interactive tool for exploring objects enriched with these annotations.

Supported platforms

Meta:Magical relies on Symbols and WeakMaps, and thus only properly works in VMs that implement ECMAScript 2015/ES6. You may run parts of it in older VMs, using Babel and polyfills, but there are no guarantees that things will work properly.

Repository Layout

This repository is organised as an aggregation of different projects that are related to Meta:Magical. The stable and active packages live in the packages/ directory, and experiments live in the experimental/ directory.


  • Interface — The Meta:Magical interface provides the basis for annotating live objects, and querying this meta-data.

  • REPL Browser — The REPL Browser allows exploring annotated objects from a regular Node REPL.

  • Mocha Bridge — The Mocha bridge allows automatically defining tests in the Mocha framework by recursively finding all examples attached to objects given a root object.

  • Babel Plugin: Meta:Magical comments — The Meta:Magical comments plugin allows annotating objects by using documentation comments, in a very similar way to things like JavaDoc, JSDoc, etc. The plugin can infer static information and automatically extract examples from documentation, which can then be automatically tested.

  • Babel Plugin: Assertion comments — The assertion comments plugin isn't a Meta:Magical feature, but was created to allow examples to be defined in a more readable way. This plugin allows compiling trailing comments in the form of 2 + 2 // ==> 4 to an actual runtime assertion, and is a good fit for examples in documentation.

How do I use this?

This section is a draft and needs to be improved on a lot, but it should give you some pointers to get started.

I am creating a library, and want to document my objects

If you're creating a library, the simplest way to use Meta:Magical to document your code is by using the Meta:Magical Comments Babel plugin. This plugin allows annotating objects by just placing documentation comments before them, as you would do with something like JSDoc:

 * This is a documentation comment.
 * It may contain multiple lines, and is formatted as **Markdown**!
 * ---
 * category : Things that do things
 * type: (String, Number, Boolean) => Undefined
function doSomething(a, b, c) {

The babel plugin will provide common metadata, such as the function's signature, the module where it's defined, the position in the source, the source itself, which object it belongs to, etc. by analysing the code for you, so you only need to provide things like category, stability, and type.

I am using a library annotated with Meta:Magical

If you're using a library annotated with Meta:Magical, you can explore the objects by loading the REPL Browser package. You need to install both the metamagical-interface and the metamagical-repl packages:

npm install metamagical-interface metamagical-repl

Once you do that, you can browse an object by feeding any object into the browser. For example, running the following in the Node REPL:

var Interface = require('metamagical-interface');
var browser   = require('metamagical-repl')(Interface);


Would give you the following output:

# Interface

Stability: 1 - Experimental
    • ECMAScript 2015

The Meta:Magical interface allows one to query meta-data associated
with a particular object, or attach new meta-data to any object
without modifying the object.

## Properties

### Additional reflective methods

    • allProperties()
      | Retrieves a categorised list of properties in the current

    • properties()
      | Retrieves a categorised list of properties owned by the current

### Auxiliary methods for querying metadata

    • getInheritedMeta(name)
      | Retrieves metadata defined in the closest parent for the interface's

    • getOwnMeta(name)
      | Retrieves metadata defined directly on the current interface's context.

    • getPropagatedMeta(name)
      | Retrieves metadata defined in the children of the interface's context.

( ... )


From: src/index.js at line 507, column 0
In package: metamagical-interface
In module: metamagical-interface

Copyright: (c) 2016 Quildreen Motta
Licence: MIT
Repository: git://
Web Site:

    • Quildreen Motta <>
    • Quildreen Motta <> (

You may navigate in the browser by using the .forProperty(name), .forPrototype(), .forGetter(name), and .forSetter(name) methods. Each of these methods gives you a new browser for the relevant object. Better yet, you can look at the browser itself to know what you can do with it, just browser.summary()!

I'd like to document someone else's objects

Attaching meta-data to other people's objects can be done safely with the Meta:Magical interface, which keeps an internal WeakMap associating objects with metadata. This also accounts for the cases where the object has been frozen, and thus can't be modified.

If you're not writing a library to provide these annotations, you can load the metamagical-interface directly:

const Interface = require('metamagical-interface');

  name: 'Object.prototype'

I'd like to write a tool using Meta:Magical annotations

If you're writing a library, you should take the metamagical-interface instance provided to your library by an external user, and just specify which version of the interface you depend on in your package's peerDependencies. Effectively, this means that you're exposing a parametric module:

// your-module.js
module.exports = function(Interface) {
    name: 'Object.prototype'
// package.json
  "peerDependencies": {
    "metamagical-interface": "3.x"

The user of this library then provides the proper metamagical-interface for you to use:


If you're using ES6 modules, this is a little bit more complicated since the modules are not first-class, and don't support being parameterised over other modules. A compromise is to export a function as the default binding, but you'll be losing static analysis and mutually recursive bindings by doing this:

// your-module.js
export default function(Interface) {
// The client
const Interface = require('metamagical-interface');
import ModuleFactory from 'your-module';
const Module = ModuleFactory(Interface);

// Now you can use Module...

Since the interface needs to keep a single WeakMap containing all of the annotations, there must be only one instance of the Interface in the entire environment.

JavaScript makes this hard for a number of reasons: modules are first class, and can be instantiated multiple times; npm may install multiple versions of a package; clients may install multiple versions of a package; code might interact with different realms; etc.

Parameterisation makes what's happening explicit, and leads to less surprising behaviours.


If you think you've found a bug in the project, or want to voice your frustration about using it (maybe the documentation isn't clear enough? Maybe it takes too much effort to use?), feel free to open a new issue in the Github issue tracker.

Pull Requests are welcome. By submitting a Pull Request you agree with releasing your code under the MIT licence.

You can join the Gitter Channel for quick support. You can contact the author over email, or Twitter.

Note that all interactions in this project are subject to Origami Tower's Code of Conduct.


Meta:Magical is copyright (c) Quildreen Motta, 2016, and released under the MIT licence. See the LICENCE file in this repository for detailed information.