JavaScript Blocks, allow you to create your own Master Blocks using HTML, CSS and Javascript. You will define the settings that a user can enter to configure your block, and create HTML templates that will read these settings and define the behavior of your block. 

Once you create a JS Block , you can drag it into your app just like any other type of block, allowing you to reuse the functionality in different apps, or in different parts of the same app, with one central place to manage your code. You can even share your JS Blocks with other users.

Your JS Blocks will consist of descriptors that define the settings and events that will be available for users of your block to configure, and HTML templates that read these settings and trigger events. 

For information on creating a new JS Block, see 

Creating a JavaScript Blocks

or help writing the HTML template, see 

JavaScript Blocks - Writing the Template.