When using PicoStack, code and data are combined within a pico. Together they represent some entity, which is responsive and reactive.
The code is written in KRL, the language you will need to learn (after having learned enough HTML, CSS, and JavaScript to get started). Since KRL itself consists of ASCII text (except in character strings and comments (where you can insert other UNICODE characters)), you can use your favorite text editor or integrated development environment (IDE) to write your code.
Besides maintaining this code on your own machine, you will probably want to use a code repository in the cloud. This post explores possibilities.