Sencha provides extensive online and offline documentation to learn any of its products. It includes quick start, guide, API help section.
Go to docs.sencha.com and select the version of Ext JS you are using in your application.
You can also configure sencha documentation offline on your local machine. Download zip file for Ext JS, CMD, Sencha Test, Web Application Manager etc. from here.
After downloading zip file for the help doc, unzip into your preferred folder and run index.html or configure virtual directory in the local web server. The following is offline help for Ext JS 6.
Learn about the recommended naming conventions in the next section.