Overview of the Construct 2 SDK
An excellent Q&A website. Also a good place to search to see if your question has already been asked and answered.
Mozilla Developer Network (MDN)
Overview of plugins and behaviors
Before developing a plugin or behavior, you should be familiar with the usage of Construct 2, or the terminology and functionality will be difficult to understand. The Construct 2 beginner's guide is a good place to start.
As you probably know, plugins define new objects in the editor, and behaviors add functionality other objects. Plugins and behaviors are surprisingly similar. Behaviors are essentially also plugins, but with the aim of affecting another object. The SDKs for both are very similar, so this guide will simply describe how plugins are made, and note where behaviors are different.
Plugin and behavior scripts are located at
Each plugin has its own folder. Plugins consist of four files:
common.js - this is prepended to both edittime.js and runtime.js in case you have code common to both.
edittime.js - defines the plugin for the editor, including all its actions, conditions and expressions.
runtime.js - defines the plugin functionality in the browser.
PluginIcon.ico - the editor loads this icon to represent the plugin.
A template for both a plugin and behavior can be downloaded here. You can copy these to a folder in the above directories to provide a skeleton starting point for your plugin or behavior.
Plugins work identically in the 32-bit and 64-bit versions of Construct 2.
All the "built-in" plugins and behaviors are also written this way. It would be useful to read their scripts - also in the above directories - to see how their features are implemented. You can learn a lot from this.
common_prelude.js - prepended to both edittime scripts and runtime scripts.
edittime_prelude.js - prepended only to edittime scripts.
preview_prelude.js - prepended only to runtime scripts.