Building a plugin in Discourse can be really simple, once you learn a couple of quirks. The goal of this post is to create a skeleton plugin and introduce you to the basics.
Your development environment
Make sure you have a development environment of Discourse running on your computer. I recommend you use the appropriate setup guide and come back when you’re done.
plugin.rb
Use GitHub - discourse/discourse-plugin-skeleton: Template for Discourse plugins to create a complete discourse plugin skeleton in your plugins directory
The skeleton is now bundled in discourse core,
rake plugin:create[plugin-name]will create a plugin using the skeleton
When Discourse starts up, it looks in the plugins directory for subdirectories containing a plugin.rb file. The plugin.rb file has two purposes: it is the manifest for your plugin with the required information about your plugin including: its name, contact information and a description. The second purpose is to initialize any ruby code necessary to run your plugin.
In our case, we won’t be adding any ruby code but we still need the plugin.rb. Let’s create the directory basic-plugin with the file plugin.rb inside it, with the following contents:
basic-plugin/plugin.rb
# name: basic-plugin
# about: A super simple plugin to demonstrate how plugins work
# version: 0.0.1
# authors: Awesome Plugin Developer
# url: https://github.com/yourusername/basic-plugin
Once you’ve created this file, you should restart your local server and the plugin should be loaded.
An important Gotcha!
If you’re used to regular rails development you might notice that plugins aren’t quite as nice when it comes to reloading. In general, when you make changes to your plugin, you should Ctrl+c the server to stop it running, then run it again using bin/ember-cli -u.
My changes weren’t picked up! 
Sometimes the cache isn’t cleared fully, especially when you create new files or delete old files. To get around this issue, remove your tmp folder and start rails again. On a mac you can do it in one command: rm -rf tmp; bin/ember-cli -u.
Checking that your plugin was loaded
Once you’ve restarted your local server, visit the url /admin/plugins (make sure you’re logged in as an admin account first, as only admins can see the plugin registry).
If everything worked, you should see your plugin in the list:
Congratulations, you just created your first plugin!
Let’s add some Javascript
Right now your plugin doesn’t do anything. Let’s add a javascript file that will pop up an alert box when discourse loads. This will be super annoying to any user and is not recommended as an actual plugin, but will show how to insert Javascript into our running application.
Create the following file:
plugins/basic-plugin/assets/javascripts/discourse/initializers/alert.js
export default {
name: 'alert',
initialize() {
alert('alert boxes are annoying!');
}
};
Now if you restart your local server, you should see “alert boxes are annoying!” appear on the screen. (If you did not, see the “My Changes weren’t picked up” heading above).
Let’s step through how this worked:
-
Javascript files placed in
assets/javascripts/discourse/initializersare executed automatically when the Discourse application loads up. -
This particular file
exports one object, which has anameand aninitializefunction. -
The
namehas to be unique, so I just called italert. -
The
initialize()function is called when the application loads. In our case, all it does is execute ouralert()code.
You’re now an official Discourse plugin developer!
More in the series
Part 1: This topic
Part 2: Plugin Outlets
Part 3: Site Settings
Part 4: git setup
Part 5: Admin interfaces
Part 6: Acceptance tests
Part 7: Publish your plugin
This document is version controlled - suggest changes on github.
for raising an error instead. Maybe we can extend that to development too? 
