0.0.2 • Public • Published

NGN Metadoc

Metadoc generates documentation metadata for Object Oriented (Class) JavaScript libraries. Running the utility will produce a JSON file describing the code. This can be used as a data source for creating custom HTML documentation (or any other output format), or for further processing.

Metadoc is released as a command line utility. It is a custom extension of the productionline build utility (from the same authors of this utility). It was originally designed to document the NGN and Chassis libraries.


Metadoc was designed to support a "code and comment" workflow. It will parse JavaScript code and extract as much metadata as possible from the code's Abstract Syntax Tree.

AST parsing creates a significant amount of information, but isn't always sufficient for creating detailed information, such as class inheritance chains.

To support greater detail, Metadoc reads inline comment blocks, written directly in the code. Comments can be used to supplement and/or override AST parsing. Comment parsing follows a style similar to JSDoc. Using a familiar @tag syntax, Metadoc provides powerful capabilities for creating fine detail in documentation.


Input Files: Event.js & Meetup.js

Source Files

Output: api.json

Output File

Getting Started

// Install metadoc
npm install -g @ngnjs/metadoc

// Run metadoc
metadoc --generate --source "/path/to/source_directory" --output "/path/to/output_directory"

Ignoring Files

It is possible to ignore files and/or directories using glob syntax.

For example:

  • --ignore "./node_modules" ignores the entire node_modules directory.
  • --ignore /path/to/**/.* ignores all files in any directory whose name starts with an dot (ex: .testfile.js).

It is possible to use the --ignore flag multiple times.


Metadoc is capable of warning developers about some common code issues/concerns:

  • --warnOnNoCode triggers a warning whenever a code comment triggers an action for which no related code can be found. This is most useful for identifying comments that shouldn't actually be in the code base.

  • --warnOnSkippedEvents triggers a warning whenever an event is detected but not documented. This is most commonly used to identify events that are considered "internal" to a class.

  • --warnOnSkippedTags triggers a warning whenever a tag is skipped. This is the default behavior, but this tag will allow this feature to be turned off (i.e. --warnOnSkippedTags false)

  • --errorOnCommentFailure throws an error when a comment cannot be processed. This is the default behavior, but this tag will allow this feature to be turned off (i.e. --errorOnCommentFailure false)

Documenting Code

The code will be automatically documented based on the JavaScript AST (Abstract Syntax Tree). However; this doesn't always reflect the true nature of how a library should be used. To accommodate customizations, the generator parses comments within the code, allowing developers to override the AST documentation with custom comment blocks.

Comment Tags

Tags can be used to modify documentation snippets. Tags use the following format unless otherwise defined:

 * @tag {[type]} <name>
 * <description>

The following tags are available:


Identifies a specific person/organization recognized as the author of a snippet of code.


Identifies a write-only configuration property.

Aliases: config, configuration


Identifies a configuration property (write-only) that also has a corresponding readable/writable property.


Identifies a class.


Marks a method as the constructor of a class.


Identifies a custom NGN Exception.


Identifies which class is being extended.


Identifies an event. See "Documenting Events" below for additional detail.

Aliases: triggers, trigger, event


Indicates the section should be included in the documentation but hidden from view. This differs from the @ignore tag, which prevents the documentation from being generated at all.


Indicates a section should be ignored from the documentation (i.e. prevents generation of a segment of code documentation).


Maintains a series of tags with additional information.


Identifies a method.


Identifies an argument/paramenter. See "Documenting Parameters" for details.

Aliases: arg, argument, parameter


Indicates the snippet is private/not explicitly accessible as a developer interface (internal).


Identifies a property of a class.

Aliases: prop


Indicates a snippet is read-only. This applies to properties.


Identifies the data returned by a method.

Aliases: returns


This is a special tag that annotates the documentation with a known task that requires completion (a developer to-do task).

Format: @todo Describe the task here


Indicates a property is only writable.


In addition to tags, there are a number of recognized flags that can be used to annotate a documentation snippet.

  • @protected Identifies a protected method/attribute.
    • @deprecated Indicates the feature will no longer be available in a future version.
    • @experimental Indicates the feature is not considered "production ready".
  • @warning Provides a warning message.
  • @hidden Indicates the feature should be hidden but not removed from the documentation.
  • @singleton Indicates a class is a singleton.
  • @interface Indicates a class is an interface.
  • @static Indicates a method is static.
  • @since* Identifies the version and/or date when the feature is generally available. This is typically used to identify new features that have been added to the original platform.

Documenting Parameters

While parameters (arguments) in JavaScript functions can have default values, there are still several cases where it is necessary to override the default parameter documentation. Most specifically, enumeration. Some methods only accept a value from a predetermined set.

Parameters can be documented with additonal detail using the following format:

 * @param {type} [<parameter_name>=<default>] (<enumerable_list)
 * <description>

The type indicates the data type, while the [ and ] indicate the parameter is optional. A default value may be supplied, as well as a description.

For example:

 * @param {String} [myParameter=example] (example,a,b)
 * This is an example parameter.

The example above describes a string parameter named myParameter. Acceptable (enumerable) values are example, a, and b. The default value is example. The description is This is an example parameter..

Documenting Callback Parameters

Callback functions are a unique type of parameter. These parameters may have their own arguments/parameters. This generator supports them using a dot notation syntax:

  * @param {function} callback
  * This is an example callback.
  * @param {boolean} callback.a
  * The first element is a.
  * @param {string} callback.b (possible,values)
  * The next element is b.

The comment above indicates a parameter is a callback method that receives two arguments: a and b. The first argument (a) is a boolean value. The second (b) is a string whose value will be either possible or values.

Documenting Events

NGN ships with an EventEmitter class (works with Node.js events.EventEmitter).

The following syntax provides a powerful way to generate event documentation overrides:

 * @fires {<arg1_name>:<arg1_type>} <event_name>
 * <description>
  • @fires is the tag. This is required.
  • <arg_name> is the optional descriptive name of a callback argument passed to an event handler.
  • <arg_type> is the data type of the argument passed to an event handler.
  • <event_name> is the name of the the event that gets fired.
  • <description>


  1. Basic Event
 * @fires {Object} myEvent
 * myEvent is fired from time to time.

this.on('myEvent', function (obj) {
  console.log(obj) // Outputs { data: 'abc' }

this.emit('myEvent', { data: 'abc' })

This event is called "myEvent", and it sends an object to event handlers.

  1. Basic Event: Named Arguments
 * @fires {myName:Object} myEvent
 * myEvent is fired from time to time.

this.on('myEvent', function (obj) {
  console.log(obj) // Outputs { data: 'abc' }

this.emit('myEvent', { data: 'abc' })

This is the exact same event as the basic event in #1, but the @fires {myName:Object} will produce a label called "myName", which represents { data: 'abc' } (payload), a known Object.

  1. Complex Event: Multiple Callback Arguments
 * @fires {Object,String} myEvent
 * myEvent is fired from time to time.

this.on('myEvent', function (obj, label) {
  console.log(obj) // Outputs { data: 'abc' }
  console.log(label) // Outputs 'event fired'

this.emit('myEvent', { data: 'abc' }, 'event fired')

The major difference is the comma separated data types ({Object,String}), which tells the documentation generator that the event will send two arguments to event handlers. The first is an Object and the second is String.

It is possible to document multiple name:type callback arguments by separating with a comma.

@fires {a:Object,b:String} would generate a label called a for the Object argument and a label called b for the String object.

It is also possible for an argument to have more than one valid data type by separating types with the pipe | character. For example, @fires {a:Object|Boolean,b:String} states that the first argument (labeled a) can be an Object or Boolean value.


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  • cbutler
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