Jenkins JavaScript Extensions

Jenkins JavaScript Extensions are extensions which contribute to the UI in Jenkins BlueOcean.
This module is used to define extension points - locations where the application accepts plugin-provided implementations.
This module is also used to process the plugin extension point implementations to provide to BlueOcean.

This module is published via npm as @jenkins-cd/js-extensions so that other plugins, external to the blueocean project, can make use of it.
Plugins can themselves make use of extension points.

Jenkins JavaScript Extensions are based on the extensibility model already established by Jenkins, based on data and views, with the ability to inherit views based on parent data types.

Jenkins JavaScript Extensions: @jenkins-cd/js-extensions module exports:
- Renderer - a React component to conveniently render extensions
- store - the ExtensionStore instance (which must be initialized before it can be used)
- classMetadataStore - class/capability metadata store
- dataType() - function for filtering extensions based on the data type
- componentType() - function for filtering extensions based on the required component type (e.g. React class)

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The ExtensionStore API is very simple, all public methods are asynchronous:

- getExtensions(extensionPointName, [filter,] onload)
This method will async load data, filter the extensions based on the provided filters, and call the onload handler with a list of extension exports, e.g. the React classes or otherwise exported references.
filter - a filter function currently the module exports the following functions - see the exported functions for the commonly used filters

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- getClassMetadata(dataType, onload)
This will return a list of type information, from the classes API, this method also handles caching results locally.

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The most common usage pattern is to use the exported Renderer, specifying the extension point name, any necessary contextual data, and optionally specifying a data type.

import Extensions from '@jenkins-cd/js-extensions';
...

For example, rendering the test results for a build may be scoped to the specific type of test results in this manner:

The ExtensionRenderer component optionally uses the classes API to look up an appropriate, specific set of views for the data being displayed.
This should works seamlessly with other capabilities.

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Extensions are defined in a jenkins-js-extensions.yaml file in the javascript source directory of a plugin by defining a list of extensions similar to this:

# Extensions in this plugin
extensions:
- component: AboutNavLink
extensionPoint: jenkins.topNavigation.menu
- component: components/tests/AbstractTestResult
extensionPoint: jenkins.test.result
dataType: hudson.tasks.test.AbstractTestResultAction

Properties are:
- component: a module from which the default export will be used
- extensionPoint: the extension point name
- dataType: an optional Java data type this extension handles

For example, the AboutNavLink might be defined as a default export:

export default class NavLink extends React.Component {
...
}

#### Enforcing specific component types

In order to ensure a specific component is returned, an extension point may also use the componentType filter - it accepts an object prototype (e.g. an ES6 class), e.g.:

import TestResults from './base-components/TestResults';
...

Extensions are not limited to React components.
The componentType filter will attempt to match returned components by a series of prototype and typeof checks to appropriately filter returned types including ES6 classes.

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By default, all @jenkins-cd/js-extensions generated JavaScript bundles will automatically preload the i18n resource bundles it finds in the
src/main/resources/jenkins/plugins/[pluginId], where pluginId is the Jenkins HPI plugin ID with all hyphen characters replaced by a path separator
e.g. for blueocean-dashboard, the path that is searched is src/main/resources/jenkins/plugins/blueocean/dashboard.

> See findI18nBundles() in @jenkins-cd/subs/extensions-bundle.js

In some situations, a @jenkins-cd/js-extensions generated bundle may depend on i18n resources that are not in the default location or not in the host plugin (e.g. they may be defined in a "common" style utility plugin). In this situation,
your plugin needs to know about these i18n resources in order to generate the right pre-loading code into the generated bundle. To tell @jenkins-cd/js-extensions about the resources in the other plugin, you need to manually define a i18nBundles
list in the jenkins-js-extensions.yaml e.g.

``yaml
extensions:
# etc ....

i18nBundles:
- jenkins.plugins.aaa.Messages
- hpiPluginId: acme-commons
resource: jenkins.plugins.acme.commons.Messages
`

> Note how i18nBundles entries can define a string or an object, allowing the loading of bundles from plugins other than the default (i.e. the same plugin as the @jenkins-cd/js-extensions` generated JavaScript bundle).